Have you tested this? The reason I ask is that unless something has changed which I don't know about, this will not be possible. Since 4.2, Google changed how android handles mounts and this is no longer possible with root (root does not have elevated enough permissions).
Hence why I created this script back in the 4.2 days:
http://xdaforums.com/showthread.php?t=2178863
*Note, I no longer support that script so do NOT use it...
...which in turn lead to this:
http://xdaforums.com/showthread.php?t=2192122
I myself use FolderMount as it works well and scripts are just a bit too clunky (hence why I ceased working on and supporting my script).
The only issue is you will need to move everything in your current music folder (which foldermount will do nicely for you) so you must always have your external media with you to play music. You cannot simply expand your current folder, its all or nothing (unless you can mount a subfolder of your music directory and music will see that? Not sure, you'll have to test).