add a composite model to an EXISTING item

Previously, I was told that a composite model could be added to an existing in-game item. The question is: How?
For context: I have Playing Card Models in-game already, and I need players to be able to "enhance" their existing cards. I've created a composite model (the same size) for a border to be applied to any one of their cards, where the border represents that it is now "enhanced". How do I do this? Essentially, is there a command that can just add a composite model to an existing item (Not a give command with the base model and composite model combined.)
Doing a replace command wouldn't work for me either, as it would essentially defeat the purpose of me using composites, rather than just new card variations for every single one 
-# There would be over 3,100 card models if I had to make a variation for every single card with every enhancement variation
While (hopefully) with composites, based on what I was previously told, I should be able to just design the base cards, then the enhancement "overlay" composites, that I could layer over the base cards to create all of the possible variants.