From a technical perspective the NPC has two parts:
1) The image when the player "chats" with the NPC
2) The animated character that walks/attacks/etc.

Now the technically superior way to implement your idea, would be to create a
framework, like we have for Tux, to add decoration to the animated NPC characters.

The easier way to do it, for a limited number of models, would be to create
additional models with the suggested modifications.