If you can find the machine that backed these tapes up, you should be able to copy the ..\Catalogs\*.U01 files from the old machine to the new media server. then shut down the backupexec services, delete the BETOPCAT.IDX file from the \Catalogs directory and restart the serices.
AFAIK, that is the only way to import specifics about tapes into BackupExec. If you can't do that, you are stuck with doing an inventory/catalog for each of the 4000+ old tapes
I don't envy you at all, and you do have my sympathy.