How can I mix body parts of 2 different Genesis 2 figures

pwiecek

For instance how Can I put Giselle 6's upper body on Keiko 6's lower body?

Richard Haseltine

The only way I know is to use a modelling application that will allow you to selectively apply a morph to a base mesh (modo and ZBrush are the ones that come to mind), then manually tidy up the join and either load the result as a new morph or as two morphs (top of Giselle, bottom of Keike) and mix them in DS.