I have never tried changing these but it looks like the materials are set within the geometry, with lines starting with "usemtl". If your prop is in pp2 format then it might have internal geometry, in which case you would edit the usemtl lines in the pp2 file its self, or it might reference an external obj file in which case you would need to edit the usemtl lines in that. In either case you would also have to change the corresponding "material" lines in the materials section at the bottom of the pp2 file. This is just theory on my part I have never tried it and I might be wrong.