You need a way to convert the graphics to a bitmap Photoshop can understand.

The main converter is Martin Wright's DXTBMP, downloadable here from the Aircraft Design Utilities page (URL in my signature file...)

You need to know how to add the aircraft.cfg entry so that you create an additional variant, and not merely repaint an existing model's textures, and to make the texture.n folder and copy the textures to the new folder.

I would recommend Abacus' FS Repainter, commercial product, since it allows you to add the aircraft.cfg entry and copies the textures to the new folder, but while you can call an external editor instead of using the default paint editor, it doesn't work with Photoshop.