It sounds like you're looking for mesh morphing, which is something that does not come out of the box. If your mesh is modest in size, you could however script this manually using the Mesh class. basically make sure that both poses have the same amount of vertices, and slowly move each vertex from the position it has in pose1 to the position it has in pose2.
If your avatar would have bones instead, this would become a lot easier.
answered Nov 17 '09 at 10:19 PM
Lucas Meijer 1 ♦♦
One more way to do it (not sure if it suits you):
I hope it helps ;).
answered Nov 17 '09 at 11:08 PM