The Puredisk documents say you can either map a drive or back it up via the CIFS Share via a "Proxy" as such. The proxy method is the way I use.
I have a print server that usually does very little traffic. It can see the files on the CIFS shares. As there is no Agent for Netapps, I use our backup service account to run the Puredisk Agent Services on the Printserver (Its a Domain Admin Account). You need an account that can read the shares eg: \\fileserver\homedrives or \\fileserver\corpdata etc.
The Backup Operator Guide Page 32 says
"UNC PATH - Use for data that exists on a CIFS network drive on a Windows client. Also use this data selection to indicate the path for a NetApp Filer."
I create various data selection under the agent and create a UNC Path Backup that points to the Print Servers Agent. The Data Selections for print server show \\fileserver\homedrives etc. and this has been working fine.
I'm unsure if its limited to Netapp only as we only have Netapps at remote sites.
Hope this helps.
The problem with all the NAS devices is that they all seem to implement the CIFS protocol a little different, and you just have to test the backup, and ESPECIALLY test the restores. You've got various flavors of linux and SAMBA running on those NAS devices, and no way for a vendor like Symantec to test all of them. Thus, NetApp one of the leaders in Enterprise NAS is the only one listed in the Admin Guides.