The meshes of the avatars included with Vizard can't be modified. If you purchase the Complete Character set there are avatars wearing short sleeve shirts. You'll also get the Max files for all the avatars. That allows you to make changes in Max and then re-export the avatar to Cal3D, the avatar format supported by Vizard.
The Vizard help has information about where to get 3D models and how to get them ready for Vizard:
http://www.worldviz.com/vizhelp/VizH...cts_online.htm