snip
Ah, well. Thanks for answering, though.
I don't think you actually need to make them nonsentient if you just want to milk them. However you may need to tame them. I know that if you get your hands on a tame male troll (either through very lucky missions to goblin sites or giving a playable civ the "use evil creatures" tag) you can shear them and they're
technically sentient. Tame sentients are weird AF tho, and in general I wouldn't advise milking a once hostile ANYTHING (tame, sentient, or otherwise) without having used DFhack to first reset their entity links otherwise you're liable to face a loyalty cascade the instant the milking job is done and the creature is loose.
That said, you could TOTALLY make a reaction that turns elf bodyparts and corpses into whatever.
Making type of vermin that stings elves and inflicts a syndrome that turns them into a caste of elf that lays gold eggs is also possible.