So, I create the closed shirt (whose pieces are separate but visually joined), import the mesh into Poser, rig it as clothing, save it as a .cr2. Then I open the buttons of the mesh in a modeler, export each button-open as an .obj and then import each as a FBM following your instructions. If the clothing somehow only covered one part it wouldn't even be an FBM. I think I get it now, thanks.