You can apply an offset to a bone, which will cause the mesh to stretch. However, if you want to apply a uniform scale, this is not possible. Here is some code showing how to apply bone offsets:
Code:
import viz
viz.go()
avatar = viz.add('vcc_male.cfg',pos=(0,0,5),euler=(180,0,0))
avatar.state(4)
#Offset the head bone, which will cause the neck to stretch
head = avatar.getBone('Bip01 Head')
head.offset(0.1,0,0)