A wizard can use False Life and Life Transference to heal someone else.
So my idea is in the first turn cast False Life (I already have the spell) to get temporary HP, then cast Life Transference to "send" that "self-healing" to another PC, and finally use False Life or Vampiric Touch (I can learn it) to recover some HP (temporaly or real ones). But I am not sure if that is possible.
Temporary Hit Points:
When you have temporary hit points and take damage, the temporary hit points are lost first, and any leftover damage carries over to your normal hit points.
Make a melee spell Attack against a creature within your reach. On a hit, the target takes 3d6 necrotic damage, and you regain hit points equal to half the amount of necrotic damage dealt.
By RAW a character which uses Life Transference or is attacked by Vampiric Touch spell would eliminate the target's temporary hit points first with any additional damage caused to normal HP after it, the spell does not care if the damage dealt was to real (permanent) or 'fake' (temporary) HP and still restores half of the total necrotic damage dealt.
Being immune or resistant to necrotic damage matters for the Vampiric Touch or Life Transference damage and regained HP, the HP being temporary doesn't.
Unless your GM house-ruled this then indeed you can turn temporary HP into actual, real permanent HP as by the book.
Life Transference is not different from how necrotic damage is handled. You should by RAW/RAI be able to cast False Life on yourself then transfer real HP to the one in need. Necrotic damage is just like fire or ice damage which one could have resistance or immunity against but it does not ignore temporary HP. No powerful divine healing or such is required just because it is necrotic instead of ice or piercing damage (as example).
You do it willing over yourself, so it surpass any effect, shield (I think that includes temporary HP)"
"Necrotic damage always surpasses temporary HP"
"If I want (but I don't), necrotic damage could rotten your body, or need high level healing spells"
These are not by RAW or RAI, no such rules exists, Either these are house rules or a big misunderstanding about necrotic damage, a GM mistake.
If the GM indeed rules that necrotic damage ignores temporary HP and shields (which makes no sense) then the use of that form of damage just greatly increased in your game, making magical weapons with that type of bonus damage, or the spell Vampiric Touch a more potent offensive option. If necrotic damage ignores in your GM's house rules a shield ask if it also ignores a full plate.