Yamaha DTXpress/DTXplorer/DTXtreme

Adding two more components using input 9/10 is the only expansion 
possibility with dedicated pads/sounds (that I know of, anyway). 
However, you could add additional pads that are Y-cabled together to 
give you more convenience. For example, a ride cymbal/pad on both 
your left and right side. Another example would be a Yammie wedge 
programmed with a crash on the edge and ride on the bow, coupled 
together with a Yammie pad, thus giving you a good crash target 
(wedge edge) and a great surface for riding (pad). I've done both.
Stephen