First of all:

If yo want to backup your NetAp, yu can use the NBU module for NDMP, it
works!

If you still try to backup (I don't know how much data), you must consider:

On both master server and client (the one that map's the NetAP) you must start
the services with an user account (not system account) The network (shared)
drives must be available to the service account that the NetBackup Client
service logs into at startup.

on a new class, go to files and add the UNC path name to the file list for the
class that to be used for the backup. 
For example this path is \\server\share



Sincerely  Javier