Should be no problem adding it to an existing Media Agent. Just go to "Expert Storage Configuration" from in the Java Console "Storage" tab. then in the resulting pop up window, select the MediaAgent in question, and from the "Start" button, select "Detect Library" As long as the device can be seen by the OS you should be able to configure the device as a library in Commvault.
Documentation Link: Tape Library
Ernst F. Graeler
Applications and File Systems