Distributed File System (DFS) that comes with Windows Server 2003 and Windows Server 2003 R2 is not cluster-aware.
You will not be able to add the virtual name of a file server cluster while defining a namespace server. Even if you try to get away by typing in the virtual name without browsing for it, you will most likely end up with an error:
The object identifier does not represent a valid object.
This means you cannot replicate between a file server cluster and a standalone file server (or another file server cluster).
To sum up, Microsoft DFS Replication service is not cluster aware and hence you cannot replicate data stored on shared storage. Even if you manage to do it, remember that storing replicated folders on a shared storage is NOT supported.