Yamaha DTXpress/DTXplorer/DTXtreme

I've got the DTX II and I've got a questions regarding sounds.

I see that there are many sysex files I can download for various 
different drum kits.  What I'd like to know is the following:  are 
these kits all just based on the internal sounds with the parameters 
tweaked?  What I'm getting at is, is it possible for me to sample an 
acoustic drum (or any sounds for that matter) with an external 
sampler and save it in some compatible format and use it in the DTX 
as the basis for a userkit?

Thanks,

Tom