I may be wrong but I think if you add the music to the same directory as the exsiting music, you simply go to the configuration page using the DroboApps Admin Utility, then click “Status” and there will be a table that tells you how many songs etc. are being broadcasted across the network. Below this there should be a link entitled “Start Scan”. Click this and jobs done.
Please note this is all from memory so specific details may be slightly out so I’ll edit this post if so when I get in tonight.