The avatars that come with Vizard include several morphs that you can use. Take a look at the following thread.
http://forum.worldviz.com/showthread.php?t=1983
If you want to use other morphs with the heads on these avatars you would need the Complete Character set that includes the Max files with the avatars. Then you could create those morphs in Max and export them.
Otherwise, you can use morphs with custom heads. In the tutorials section of the Vizard docs, go to Avatars --> Custom Avatar Heads and take a look at the tutorials in there.