I don't believe there is a way to limit the bandwidth used by a NetApp filer during an NDMP backup.
I know that the official NetBackup 6.5 System Administrator's Guide says that bandwidth throttling is not supported on Oracle, DataTools-SQL-BackTrack, and MS-SQL-Server policy types. However, since it also says that the bandwidth throttling is enforced by the NetBackup client software, I would also think that NDMP policies would be incapable of using bandwidth throttling (since there is no NetBackup client software installed on an NDMP host such as a NetApp filer).
The NetApp filer will try to push data out it's LAN port as fast as it can or as fast as your NetBackup media server will accept it, whichever is slower.