Because NDMP is used to backup your NAS, then backup software cannot encrypt the data. Even though you are using NetWorker, the actual backup command (i.e. tar or dump) is run on the NAS itself.
Based on the information that I found (see below links), your options are to use hardware encryption at the NetApp or at the tape library level. Please contact their technical support for assistance on this matter.
Questions about encryption
If the NDMP data is directed to a NDMP Remote Server-enabled MediaAgent for data protection or auxiliary copy you can software and hardware encrypt the data. For NDMP data sent directly to a filer-attached library only hardware encryption is supported. Filer direct Hardware encryption requires a 3rd party key management system.
NetApp encryption solutions continue to protect your data when it is backed up to disk or tape.
Quantum Scalar i500 User Guide, Chapter 7, page 171, “KMIP-compliant Encryption Key Management“