When I was swimming around in atm7, I noticed aquaculture fish don't drop spirits, but ars nouveau mobs do. Haven't checked mana and artifice yet.
How do I add mobs to the loot table?

You can PR in some spirit files, but in general you just use a datapack with json files under data/malum/spirit_data/entity/

You can see how this fella did it here 574289b