Yes. Version 5.1 (or 5.1.1) will allow you to select a portion of local storage for the VSA, and keep the remaining local storage for already deployed VMs. You should then be able to migrate your VMs from local storage to the VSA shared storage once the cluster is created. Finally, there is the ability to reclaim the rest of the local storage and hand it off to the VSA to increase the size of its shared storage.
One caveat - VSA 5.1.x needs to increase the VMFS heap size to its maximum value. If this has not already been done on the hosts, the VSA installer will look to reboot the ESXi hosts for this change to come into effect. This means that your VMs will have to be shutdown.
If you have already increaed the heap size to its maximum, then the reboot step is not needed.
This link has a lot of useful info pertaining to the VSA.
HTH
Cormac
