Nintendo of Japan have confirmed that the side pad add-on for the 3DS does exist, and it will go on sale in December for 1500 Yen (about $20) windows 7 software kostenlos herunterladen. It looks a little bulky, but hopefully it will still be comfortable to hold anyway. A list of games has already been confirmed for compatibility with the device:
Monster Hunter Tri G
Resident Evil: Revelations
Metal Gear Solid: Snake Eater 3D
Ace Combat 3D: Cross Rumble
Dynasty Warriors VS
Kingdom Hearts 3D: Dream Drop Distance
Games that are compatible with the device will display this icon on their box art: