For example, you wish to modify the female nude. The original file looks like the example 1. What you need to do is rename every group that is supposed to be using a different material to a different name. Make certain you don't rename every occurrence of a group to a different name, but only those using different materials. Plus you will need to create groups for those parts that are just identified by a usemtl statement.
Easy way is to change the usemtl statement into a g statement.
Make a list of what groups you create, as you will have to reverse this process later. Now you import the OBJ and modify it. When you are done group all the parts of the figure and export the figure. Now you get to edit the OBJ again, otherwise poser will import a horde of funny colored parts. So, edit all the renamed groups back into the original name and insert the usemtl statements.