While the house of Mario is yet to officially confirm the existence of the add-on itself, a posting on theofficial Nintendo Japan website not only confirms it's real, but spills all of the details. According to the listing, the peripheral will be called the Nintendo Expansion Slide Pad and will be available on December 10, 2011, for ¥1,500 ($19.46).
The Nintendo Japan website also lists six games that will support the Nintendo Expansion Slide Pad: Metal Gear Solid Snake Eater 3D, Kingdom Hearts 3D, Resident Evil: Revelations, Ace Combat 3D: Cross Rumble, Dynasty Warriors VS, and Monster Hunter Tri G.
Exact specifics on how the Nintendo Expansion Slide Pad connects to the Nintendo 3DS are unknown at this stage. Interestingly, though, the Nintendo Japan website says that the peripheral will require one alkaline battery to operate, which suggests it will run off of its own power source.