Look at the 'Import' section in the Catalog chapter of Admin Guide I:
AdminGuide I Windows:
ftp://exftpp.symantec.com/pub/support/products/NetBackup_Enterprise_Server/290203.pdf
Admin Guide I for UNIX and Linux:
ftp://exftpp.symantec.com/pub/support/products/NetBackup_Enterprise_Server/290201.pdf
The 1st phase of the import scan the tapes for image header info (do this for ALL tapes before starting phase 2). Phase 2 builds the file info and takes time - at least as long as the backups took....