The only available directives available for NDMP via the drop-down list in policy backup selection are (for us anyway) NEW_STREAM, UNSET and UNSET_ALL.
Any of the SET commands are typed in "manually" per the Admin Guide Chris pointed you to and can be protocol or vendor specific:
"...
I
n general, the syntax of a SET directive is as follows:
SET variable = value
Where variable is the name of the environment variable and value is the value that is assigned to it. The value can be enclosed in single or double quotes, and must be enclosed in quotes if it contains a space character.
..."
I would also direct you to the document mentioned in the Admin Guide Chris pointed you to:
Veritas NetBackup (tm) for Network Data Management Protocol (NDMP) Network Attached Storage Appliance Information
http://seer.entsupport.symantec.com/docs/267773.htm
There is a short section under
NetApp that deals with "Image Backup (smtape)"
NDMP - "an open standard
protocol for enterprise-wide backup of heterogeneous network-attached storage"
http://www.ndmp.org/info/faq.shtml
SnapMirror2Tape - a
method of backing up large NetApp volumes (faster than traditional full NDMP saves)