Editing existing textures without remapping them is actually quite simple: Open the .mat file in UTF Editor, export the texture files to .dds from its texture library, edit the .dds files in Photoshop or any other image editing program which supports .dds, then import them back into the .mat file using UTF Editor and save it.

However, since existing Phantom snubs are low quality imports and have rather awful texture mapping (it's basically one solid black texture with a logo applied to the whole mesh), retextures of the same models would not be suitable submissions and I suggest you go for fresh models with original designs instead.