Hi
The TD6 is basicaly a sound generator so quite impossible for you to add your own samples as you could normally do with a sampler (Roland SPD-S for instance).
You can just edit your internal sounds, that's all. Although you don't have nearly as many editing capabilities over sounds as in the higher TD8, TD10, or TD20 models.
The only alternative for any sound extension: trigging your module on your computer via midi output and using a virtual drum software like BFD, Addive Drums or Easy Drummer...etc.