Is their a way to use the Player character models for custom npc's?
I believe you can just name a custom player character you made in the "Custom Player Character Definition Name" field. It pulls the models used in creation process. It doesn't appear that the preset player characters can be used, since their names don't turn green. I suppose you'd have to remake them if you wanted to use them as custom NPCs.
I also noticed you can't play as a custom player character if it's being used as a NPC, which makes sense.