Be aware that some modellers will ignore the grouping information in an imported OBJ - Anim8or for instance does this. It means that the result won't be conforming unless you regroup it. However, if "changing the shape slightly" is your goal, wouldn't a morph be a simpler method? You could do this right in Poser with the magnets. Alternatively, if you want to cut bits off, try a transparency map.