I'm trying to remember the name of a feat that allows you to convert power points to spell slots and vice versa
Dragon vol. 349 is the source of pretty much all of the available options here. All of them are painfully inefficient.
Cannibalize Spell—spell slot to power points inefficiently
Cannibalize Spell allows you to trade spell slots for power points. You cannot trade spells of a level higher than your maximum power level (so to trade a 3rd-level spell slot, you need to also be able to manifest 3rd-level powers), and the trade is a spell slot for 1½× the spell’s level, which is a pretty inefficient trade (since powers of a given level require nearly 2× that level in power points).
The best use-case might be duskblade, since those get an unusual number of low-level spell slots, and the inefficiency doesn’t set in until you’re dealing with 3rd-level spells. I guess you could try something like a 5th-level duskblade/10th-level war mind with a generous DM who allows Combat Casting, arcane channeling, and quick cast to be more psionic–magic transparent than they really are. The need for both Intelligence and Wisdom hurts there, though.
Cerebremetamagic—inefficiently pay for metamagic with power points
Cerebremetamagic allows you to prepare (only; sorry sorcerers) a spell with a metamagic without having to use a higher-level spell slot. Instead, you expend power points equal to twice the level the metamagic’d spell would have used—not twice the spell level adjustment of that metamagic, but twice the total effective spell level that spell would have had. You still need to be able to cast spells of the level it would have had, so this isn’t a way to use metamagic on your highest-level spells.
The given example is preparing empowered magic missile in 1st-level spell slot by expending 6 power points. Since you effectively have to pay for the spell’s own level twice (once with its regular spell slot, and again by including it in the power point cost), this is remarkably inefficient.
And on top of all that, it only applies to a single metamagic feat selected when you choose this feat.
There isn’t really a good use for this. You have to overpay pretty substantially no matter what you do. It becomes more efficient with lower-level spells paired with high-cost metamagic, so I guess that would be something, but the need to still be able to cast spells of the adjusted level normally badly limits the value of that.
Psiomancer—power points to one prepared spell somewhat inefficiently
Psiomancer is, for the most part, strictly superior to Cerebremetamagic, and it’s also a fair bit more efficient than Cannibalize Spell. It allows you to use power points to prepare spells, whether you use metamagic or not, and the inefficiency is 2 power points, instead of half (rounded up) of the spell level in question.
But like Cerebremetamagic, Psiomancer has to be used to prepare spells; sorcerers are out of luck. And like Cannibalize Spell, this feat requires that you be able to both manifest powers and cast spells of the given level.
Unfortunately, on top of all of those limitations, Psiomancer can only be used to prepare a single spell each day, which is the only reason it isn’t strictly superior to Cerebremetamagic.
Realistically, Psiomancer has probably the best effect of these feats. At low levels, Cannibalize Spell is better, but that quickly changes. At higher levels, the “transaction cost” of 2 power points is probably fair. The problem is, you can only use it once per day. That doesn’t work well at all. And at least Cannibalize Spell has some weird build-around options where you focus on converting low-level spells to power points; Psiomancer doesn’t even have that use-case.
Psiotheurgist—huge caster and manifester levels?
Finally, 349 does have another feat of note for a cerebremancer: Psiotheurgist, which allows you to add your caster level and manifester levels together for a given Spell School–Psionic Discipline pair. Combined with the cerebremancer prestige class, that potentially could mean rather high caster levels and manifester levels, which is nice, ish.
Mind Mage—psi-spell prestige class
Dragon vol. 313 had a series of “Psi-Spell” feats, which were mostly garbage. If you had two of them, and could cast 3rd-level arcane spells and manifest 2nd-level psionic powers, you could enter the mind mage prestige class. This is basically a cerebremancer that loses another two (!) levels from each side in exchange for actually having class features. Some of these are half-decent, but none are worth the cost. Two are relevant here.
Compensation at 2nd level allows you to pay power points to avoid metamagic spell increases, or expend spell slots to avoid metapsionic power point costs. The PP-for-metamagic version is inefficient (3 PP per spell level), but the slots-for-metapsionics version is appropriately efficient (each spell level is worth 2 PP).
Then focus of discipline at 7th level allows you to spend an astounding 8 power points per spell level to reduce the spell slot needed to cast a spell. So you can can, per the example, prepare cone of cold in a 4th-level spell slot instead of a 5th-level spell slot for 8 power points. Please don’t ever do that. On the flip side, ridiculously, he can expend a spell slot to reduce the power point cost of a power by 2 per spell level—no staggering 25% efficiency there!
Anyway, some of mind mage’s other features allow you to increase your DCs and your caster/manifester level, partially undoing the damage wrought by the lost progression. If your psionics comes from being an ardent, and you have Practiced Manifester, this could actually wind up being worthwhile if you can cheese your way in on a single spellcasting level (Precocious Apprentice and a visit to the Frog God’s Fane would be one way, if I recall correctly), since spell slots are fairly efficient at powering your psionics with the mind mage. But you would basically never actually use your spells, except maybe with the awesome force touch feature, which makes it kind of different from what you’d think such a character would be like.
Psithief—steal powers and power points
The only notable psionic–magic feat that I can think of actually published in a book is Psithief from Complete Scoundrel, which allows a spellthief to steal power points and powers. Spellthieves are still really low-power, and unless your campaign has remarkably common psionics, this isn’t worth a feat to one.
Conclusion—can’t be made worth it
None of these feats does enough to salvage the cerebremancer or psychic theurge, sadly. Like mystic theurge, they’re pretty much pure trap. The best case scenario is probably a 1st-level cleric/2nd-level ardent entry with Mad Faith and Practiced Manifester, which I guess works out OK enough, but not as well as it should considering the level of cheese going on there. And even so, I probably wouldn’t bother with Cannibalize Spell or Psiomancer. Psiotheurgist, maybe. The mind mage could be cheesed into some utility, mostly by using early entry to limit how much lost progression you have, and using ardent to make the lost progression matter less, but otherwise is garbage, and even if you do it all you get is to burn arcane power to fuel psionics.
Homebrew Suggestion—one of the first things I wrote for 3.5e
I did write a homebrew cerebremancer that incorporates effects similar to these feats (but with fewer limitations), which may work better for you—it certainly has for me, though it has been a long time since I or anyone else I know has used it.
I doubt that this is what you were thinking of, but the spell dweomer of transferrance can convert spells to power points and the spell mental pinnacle gives you 3 pp per caster level and a restricted selection of powers. Both are on page 220 on the EPH.