Will, a collegue of mine was able to migrate the extents of the two legs but the expandable space of the virtual volume did not change. So it seems the best way is to expand the backend lun. I have brought this up to the product management group
What is the proper way to expand a distributed volume in a metro configuration?  I know that you can not do mobility between two distributed volumes, or between a distributed volume and a non-distributed volume.   Any thoughts on how to do this?
How to expand a distributed volume