I believe that 6.0 agent is able to backup the DFS data.
in 6.5, there are below options to backup dfs data
Workaround 1
follow the technote
http://seer.entsupport.symantec.com/docs/290900.htm
Workaround 2
Another option is to use a hub server where by you replicate the data to another server, turn off replication, then back up that server.
Workaround 3
Use an advanced client offhost method to snap the volume.
-Rajeev