ZSpheres are an object generator. As such, there are certain things that you can and cannot do on a polygonal level while the mesh is still connected to the ZSpheres (while it’s a preview).
Since you’re already talking about other software here, let’s explain this in those terms. NURBS are also object generators. As long as the surface remains a NURBS surface you cannot manipulate it on a polygonal level. If you wish to restructure the polygons themselves, you must make the object editable which in turn removes the ability to modify the model’s shape via splines.
ZSpheres are no different in that respect, although you CAN sculpt the shape of the preview. But you cannot modify the underlying structure of the polygons. As far as sculpting the shape of the preview is concerned, that information is stored as a morph target. If you do something that changes the point structure of the preview, that detailing will be lost. Trying to import a modified mesh from another program would do that. In short, what you’re wanting to do is not possible. If you wish to be able to repose the model within ZBrush, then you need to leave the model connected to ZSpheres which means that it will still be an object generator and have the limitations that go along with that. If you wish to manipulate the object’s polygons beyond what can be done with the preview mesh, then you need to actually skin the model. In that case, the ZSpheres can no longer be used to pose it.
Generators and Polymeshes are both extremely powerful ways to arrive at your finished result (good artwork). Each has advantages and disadvantages that the other lacks (this holds true for every software package). Understanding the differences between the two will help you choose the option that best suits your needs.
I hope that clears the air a bit. 