in my experience when you start doing stuff like that: merging groups and then re-splitting them - you get all sortsof crazy stuff happening like hedgehog-like models with exploded spiked polys.
any change to the winding order of the .OBJ and poser blows it apart.
any change to a SINGLE vertice and your morph wont load.

so no, i wouldnt recommend it. also, even if the morph tool did span groups, it would take you eons to morph fine detail with it. its also not good at working on morphs near the centreline of the object since when you mirror them youll get a nasty crease.
what it really excels at is after you set up a render and do your previews, you can go in and poke around at the model a bit and make it look more natural before you do your final render. luckily with V4, the group lines are not where you would think: for example her thigh/shin group line is a few inches ABOVE her knee, letting you edit the knee joint a bit with the morph tool if you are careful.

you could even pull off something like a bit of a crease/skin roll effect on the abdomen of a bending figure, for example. morphs can be mirrored and saved as dials very easily.