I already created the vmt and vtf files, but I can't seem to figure out how to add it as a skin. The model is 'models/props_bts/glados_ball_reference.mdl' which I ported to portal 2. Any help?

Changing the amount of skins is one thing. It requires to decompile the model, to edit the .QC file adding the new skins, etc.

But to edit an already existing skin is other thing completely different. It is easier and faster. All you have to do is to keep the original name of the texture files (.VTF and .VMT).

In your case, there are 4 skins already, named something like "glados_ball_01", "glados_ball_02" and so on... Just edit one of those and you'll have you own custom skin for that model. I guess you already grabbed them and placed under your materials/models/props_bts folder, right?

The easiest way to add more skins is to duplicate the model and hex it. Now instead of one model with four skins you have two models with four skins each, for a total of eight.

Thanks to both of you!