I guess the NetApp Technology Alliance Program is the way to go:
https://www.netapp.com/pdf.html?item=/media/7248-alliance-program-guide.pdf
The internals of the NDMP spec is public (https://www.snia.org/ndmp& https://www.snia.org/tech_activities/standards/software/ndmpv4) but I did not find any implementation details about the CAB protocol extension (https://docs.netapp.com/us-en/ontap/tape-backup/cluster-aware-backup-extension-concept.html) so I guess NetApp developed that internally.