Well, how do you think he was able to seduce Anakin over? He did have some knowledge about being able to transfer life force between beings. Hence his spiel in the movie about “darth plageous the wise”. The lore wasn’t that deep than, but it seems seeds were atleast there.

It’s the unfortunate thing about being able to flesh it out after, kinda makes the stuff seem “too convenient”. Was Lucas thinking THIS far ahead, unlikely, but it did seem he had some idea of life force transfer in his head at the movie time.

The movie does also imply in a very not obvious way and a bit of a stretch that padmes life force was taken to save Ani as well.