Here’s another idea: A hypnosis spell that will make any enemy drop his/her weapons (with the exception of bosses like the Goblin Chief and the Succubus) and become your feeder slaves in a special Red District building called “Club Feeder”.
Realistically I could never see this happening as you’d have to tame the enemy and teach them to feed you and all that jazz. Not to mention the governing body of Turici would want to try and get rid of all the monsters in the city for the safety of the townspeople, so they’d never allow that to happen.