You can use animatable joint centers to change the rigging on any figure. 

Here is a youtube video tutorial by Chuck Taylor that shows how to do so:

http://www.youtube.com/watch?v=ogVjot24X20

 

The custom morph needs to be sculpted on the figure in its default shape.

If you're using zbrush, then go to Figure > Export Figure Mesh to GoZ. Make sure to leave "Export Posed" unchecked so that it sends the figure to zbrush in its default pose/shape.

You can send the entire figure and then hide whichever parts you don't want to work on in zbrush.