How are you getting the mesh into ZBrush? Are you exporting from Poser using GoZ, or are you starting with the unified, default character OBJ? 

If you use GoZ to export the OBJ into ZBrush, the issue you may be having when going back to Poser is that ZBrush may have automatically welded the vertices at the group boundaries. As a result, the vertex count is different than the version that was exported from Poser, and that may be why you are being prompted to save it as a prop in Poser.

Generally, you probably don't want to just morph the head, because then it may no longer match up with eye and mouth positions (which are separate groups).