Disclaimer: I'm using my phone to post this and its a bitch so I can't be bothered to google stuff and double check things, so please forgive my incomplete information. It's justbeen so long since I messed with this stuff that I can't remember the details.
It IS possible to load your own samples onto the ddrum. I had an older model, but I'm sure the model you have would work too. Be warned though, that no matter how sweet your custom sample is, you'll be hard pressed to get it sounding as good as the preset samples. The stock samples have some sort of file compression method that keeps them sounding good, while any custom samples you load have to be first converted to midi data packets and go through some different compression method I think, and this somehow makes them sound kinda crappy. Almost like the're played through a weird filter or something.
It may be a tough search, as it was tough as nails for me years ago, but there is a super old program that converts wavs to midi data filed, which can then be loaded onto the ddrum. I think its actually called "wave to midi" or something, and it looks like it was made for windows 95. iirc, it also does all the transferring to the module for you. I remember having a bit of trouble figuring it out, but I think it was something to do with needing to connect two midi cables (in and out) or something like that.
It sucks that I don't have internet on my computer anymore because I have the installer for the program on my pc. I just have no way of uploading it. If you can't find it after an exhaustive search pm me and I'll see what I can do about transfering it to my phone and emailing it to you, if that would work.
Not sure if this helps, and hopefully you can find that dinosaur program or something equivalent. Also hopefully your results are better than mine were. I'd always load a kick that sounded supreme on my daw, only to be dissapoint with the sound of it coming out of the module, and reverting back to preset kicks.