Yamaha DTXpress/DTXplorer/DTXtreme

I have a question that I am sure has been asked but I searched the 
archives and couldn't find an exact answer.  I just bought my first 
drum set (a dtxpress iii) and already have maxed it out.  If I want 
to add more pads I know I need another module.  Is a mixer what I 
then need to make them work together to hear them either through 
headphones or an amp?  Sorry for the stupid question but I work with 
consumer electronics and not at all with "pro" gear.
Thanks,
Ed W