You can add new material to an existing archive as long as there is space on the archiving medium. When opening an archive to add entries, use the table of contents on the archive. For VTR archives, use the OTOC to add entries.
To append entries to an existing archive:
(a) Library Type box (b) Clip Library box (c) Backup Set box (d) Archiving Mode box
Images courtesy of Technicolor (formerly Toybox) and Das Werk