Wouldn't that be great if someone figures out how to work out a rather easy and not incredibly time consuming method to enhance the FG mesh and then use it with millennium figures in Poser. I'd pay for a tutorial like that and I presume many people also will.... :)

In order to get it to work properly, a custom neck prop would have to be created so that it blends properly from the FaceGen head to the millenium figure. And that would also mean that multiple neck replacements would be needed (one for each figure you want to use). 

Aside from that, the geometry in the default FaceGen head isn't as detailed as in the millenium figures. No nostrils (they are flattened), and the mouth and eyes aren't poseable. That probably explains why most images that use the FaceGen head are usually from straight on, and have similar expressions.