Map Mouse Button to Key
 

I have a Logitech MX700 mouse with lots of buttons and a keyboard with few extra buttons. Instead of using the two thumb buttons for back & forward, I would like to assign them XMMS back & forward. Is there any way I can assign these buttons a keycode?
If you know another way, you can tell me it too. I'm not really set on using this keycode method, but it sounds easier. Then I can use the xmms-itouch plugin.