Recently, to my dismay, I discovered that my longstanding understanding of the Dire Weasel's Blood Drain ability was incorrect. As Blood Drain deals Constitution damage, the weasel does not gain temporary hp. Unfortunately, the temporary hp gain is rather important in a couple of modules I run recurrently, and so I am seeking a feat from somewhere in 3.5's vast body of first party sources (including Dragon and Dungeon) that would allow the ability damage to instead be ability drain (and thus grant 5 temporary hp/round per victim attached to). Does such a feat exist? I desire a feat, as opposed to other sources, because replacing the Dire Weasel's Alertness feat with such a feat would impact the use of such a monster least, while e.g. spells, magic items, etc require either additional HD in class levels for the weasel or an outside source (i.e. a friendly druid) currently not present in the most important weasel-based module I run.
No feat turns ability damage into ability drain
I've a document that I compiled for personal use that has, I think, all of the official feats, including feats from licensed sources. It runs over 2,500 pages, but that's including all my notes. Nonetheless, all told, it probably contains, like, four thousand feats, and while I won't claim that I've actually caught them all, I'd like to think I almost have and that any remaining omissions are from sources so obscure no one has them anyway.
With that in mind, I can say with a great deal of confidence that no feat transforms a creature's ability damage into ability drain. In fact, no feat even comes close. The effect ability drain is, of course, mentioned by several feats like Detach (Savage Species 32-3), Empowered Ability Damage (Libris Mortis 26), Inheritor (Dragon #315 70), Pawn in the Great Game (Dragon #354 56), Shriveling Touch (Ghostwalk 38), and Tainted Construction (Expanding Your Mind Web column "Subpsionics"), but ability drain is usually mentioned in the context of bonuses to resist ability drain, restrictions on the use of ability drain, or the creature gaining a unique ability drain effect—often with such a feat having very strict prerequisites (q.v. Shriveling Touch).
So, yeah, I'm almost sure you're sunk. I mean, it's possible, for example, for a psion to use the power astral construct and, with the feat Tainted Construction, grant that astral construct an ability drain attack… and were that a psion to possess a sufficient Craft (sculpting) skill bonus, the psion could make that astral construct appear to be a weasel or dire weasel. But that's exactly not what you want.
The closest you may get is the domain feat Death Devotion (Complete Champion 57)—which seems to fit well with weasels as described in your campaign—that allows a typical weasel 1/day for 1 min. to cause foes struck by its melee weapon (its bite likely qualifying) to make Fort saves that, if failed, causes a foe to gain a negative level. Even the effect of the Death Devotion feat, however, isn't true energy drain, so while the weasel, during that minute, will shred the heck out of, like, 10 commoners (and, tomorrow night, they'll become wights—weasels can still be jerks!), your weasels still won't have the special attack ability drain.
I suspect you must venture beyond official material for your weasel needs.