Yamaha DTX Multi-12 drum pad SW/HW hack
I don't think the software can be 'tweaked' since it's built in some flash/EEPROM and cannot be edited.

The most easiest way is probably to use an external sampler, assuming your Yamaha DTX has MIDI output. Modern samplers (or even by using a laptop with MIDI interface) can give very long samples.

But of course this would 'spoil' the project.