Originally Posted by Te Mirdala Mand'alor
I want to add a new armor model without replacing any existing ones.
This is not possible to do, as far as I am aware. An armor base item type has a certain body variation value set for it in the baseitems.2da file. This body variation corresponds to a set of columns in the appearance.2da file, which list the name of the model and texture a creature using that appearance type will get when equipping an armor with that body variation.
Since the game won't read any new body variation columns added to the appearance.2da
file you can't add new body variations. Thus you have to overwrite one of the existing ones to add a new armor model.
(The best you could do in that regard to make it more exclusive would probably be to make the change only to specific appearance types; for example make your new armor look only apply for a party member or the player and no one else.)