Sorry, there's no external speaker jack for that amp. You would first have to find out what ohm rating the amp has, and then you would literally have to go inside the amp section and rig up some sort of wiring to get you to another cab. Bad idea and not worth it. You would be better off to save up for a stronger rig if you need more volume.
If you're gonna be stupid, you gotta be tough. - My Grandmother