About skins like skin1, skin2, etc.

Well im pretty sure that almost everone knows that their is a few different kinda skins in almost every tf2 ragdoll.
As an example we can take scout and he has four skins.
Blue, Red, Ubercharged Blue, Ubercharged Red
But could someone please explain, how do you exacly add more skins to that list?

Bump

wrong section and decompile the model and change the skin families