Seeing how DJ Hero is played, and how small the DS' touch screen is, no, not exactly possible. But it could be a whole new port, where you had to wear 3 finger gloves, that have like little pointy things, to act as the turntable buttons.
I think it can "hopefully" be done via touch screen...problem is only one input or "touch" can be made so you can't hit 3 colours at once. I'd like to see this done, but I kinda doubt it...but prehaps they'll make another dumb accessory.
I hope that the next dumb accessory would be compatible with a DSi. :-< Maybe stick it in through the charging hole. I dunno, just hopefully.
Doubt it. But if they did that would be funny. That would be bad if you really needed to charge but you were almost finishing a song and you really didn't want to quit.