You'll need to upgrade your TSM environment to 5.4 first then this is possible. Here is an excerpt from a what's new in 5.4 document.
"Off-site vaulting of NDMP backup tapes
You can now create copies for on-site or off-site vaulting of NDMP-generated backup images (NDMP backups) that reside in a storage pool whose device class is type NAS. These copies can be used to improve disaster recovery capabilities. Tivoli Storage Manager now supports the backing up of primary storage pools that contain NDMP backups to copy storage pools. The copy storage pools can be used to provide off-site vaulting copies, and to restore any damaged primary storage pools or to restore filer data. The copy storage pools, created for NDMP backups, can also be used with a comprehensive recovery plan that can be automated using the Tivoli Storage Manager Disaster Recovery Manager.
NDMP-generated data can be transferred to other storage media. The transfer can be done toreclaim unused space or to migrate data to new storage media."