Yamaha DTXpress/DTXplorer/DTXtreme

You cannot add sounds to the DTXpress, only alter the ones in it.  

The easiest way to try out some new sounds is to download other people's kits into your module.  There are quite a few in the files section here and more at http://dtxpressions.com/

You have a lot of flexibility in altering the sounds although if you've tried all 122 snares and 120 electric snares and nothing gets close then maybe you're stuck.  However, you can usually get what you want by altering the decay, tuning, reverb and cut-off frequency.  "Layering" two sounds can make a big difference to the sound, although quite a lot of the snares are layered sounds anyway.  Something with a fast attack (i.e. high decay) layered with a bigger, more resonant sound tuned low might work for you.  You don't have to layer two snares either - I have mixed a tom with a snare to get a specific sound.

Crossfade can be useful, although probably not for your rock sound.  I have used it to get a brush sound on gentle hits and a snare on harder hits which can be useful for certain types of music.

Anyway, I think downloading a few kits would be the best starting point.  Quite a few kits on the DTXpressions site claim to be big sounding rock kits.

Keith.