Been playing around with the transform mechanics lately. I find that a lot of characters just sort of laugh off being given a potion to drink. How do you make them drink if you cant use forceful actions on them?
In the Content Options>Gameplay there is a setting that prevents NPCs from spitting out those potions.
If you turn that off, they shouldn't reject the potions, even if they don't have that test subject fetish.
Without that option disabled, I think most NPCs will reject the potions, unless the have the Test Subject fetish. You can enchant a Mystery Kink potions and give them that fetish or just enchant an item with that fetish and equip it on them.
As others said, the unique (story) NPCs can't be forced to transform.
I can't remember if offspring need that fetish before you can transform them or not, but I think they do need it even if you have disabled rejecting potions.
If nothing else works, you can go into the content settings and adjust the Test Subject fetish to be Always, that should give every NPC that can be transformed the transforming fetish when they are generated by the game, it won't affect any NPC already in the game, but new NPCS would have it.