Is it possible to add a slider to loot generators that will cause an item to be generated and then enchanted as if it was enchanted by a enchanting table at the selected level?

Hmm... Theoretically, that's what the artifact generation tag is for. Have you tried to see if that one does what you need?

The artifact generation tag can't control which items will be in the chest and I can't control how strong the enchantments are. I'm trying to make the enchantments random so it would be nice if I can make certain items be enchanted at level 20 and others at level 30.