Yamaha DTXpress/DTXplorer/DTXtreme

I just purchased a DTXPRESSIIISP but it has not arriced yet.  A
question that has been nagging at me..... 

Is the DTXPRESSIIISP in any way expandable? ....
as far as something like adding a cowbell trigger without sacrificing any of the standard
triggers and sounds that come with the kit as is from the factory? 

Or are all the trigger points used up already with the hardware provided?

Also, does anyone out there have any suggestions for setup that are not already in the 
manual?

Thanks, THC