You can do it with UVMapper Classic, unless the Mac version is very different from the Windows one. You don't even have to assign materials - just uncheck the 'Export Materials' option when saving the model. Your OBJ will then have no material definitions, and Poser will assign everything to the default material. You could even do it with a text editor. :) Go through the OBJ and delete every line that starts with 'usemtl'.