You can, simply backup the filesystem/ directory where it resides. However, this would be an issue if you need to recover it, when the Netbackup server requires a Catalog recovery.
The DR file will be required for the recovery, but, it is on a tape which you will be unable to recovery easily from. It is possible to rebuild the DR file if it is lost, however this is likely a time consuming process. Likewise, you can read a NBU non-multiplexed tape using tar. It is probably better to just email the DR files elsewhere.
Martin