How to add/Edit a player model

Essentially im trying to figure this out to help with a Steven Universe Mod That I follow, cause i really wanna see this become a feature.

But i was wondering if there is anyway to add a model to the player model, or like fuse it to the body after an event without using a plug in; as its for a Steven universe Mod its for Gem Placement on Various Body Parts. if anyone has any advice or could help me with this, please let me know, thanks.