Textures for armor items always are defined within the NIF file of the item itself. You need to create 3 copies of the boots' NIF, open each one with NifSkope, and redirect the texture setting to the different variant (DDS file) it should be.
The boots likely consist of one or more "NiTriStrips" or "NiTriShape" blocks, each containing a "NiMaterial", "NiTexture" etc. block for material, texture and other settings. The information defined inside the "NiTexture" block is what you'll have to modify for the mesh to use a different texture.
Once you created the 3 new boots NIFs you can 'duplicate' the one boots entry from the mod inside the CS the way Lanceor described, and change the NIF file used by each copy to the appropriate new one.
I don't know how they're added into the inventory, but if it's a scripted approach, it should be rather easy to expand it to add the other 3 sets as well. And if it's not scripted, well, that shouldn't cause much trouble either in fact.