I have been looking for aways to change the existing geometry of Norman or similar rigs. I found this tutorial online. I am sharing it with all of you. I persionally have not yet tried it, but intend to try it very soon. Shall post my responses once I ma done.
Link:
http://animationbuffet.blogspot.com/2008/03/norman.html
Heres another link explaining the same stuff although it looks a bit different.
http://aauanimationclub.blogspot.com/2009/10/part-one-of-norman-modification.html
Animation and Creativity 