So first off, you cannot install VSA on a single node and then add a second node. Similarly, you cannot grow a 2 node VSA to a 3 node VSA at this time.
What you do however is what is termed a 'brownfield' install - this is where you can install VSA on hosts and preserve exisiting VMs.
When you install the VSA, it will see that some of the local storage is being used by existing VM(s) and will not consume all of the local storage for the shared NFS datastores.
Once the VSA is installed, you will still have a VM on local storage and you will have the shared NFS datastores from the VSA.
At this point you can migrate the VM from local storage to shared NFS storage, and reclaim the space what was previously used by your VM on the local storage, i.e. grow the NFS datastores to consume all of the local storage.
This can only be done with the 5.1.x release however - recommendation is to use the latest 5.1.1 version.
HTH
Cormac