Mii Maker lets you create Miis, like Wii Mii Channel. Here you can transfer Miis locally from other 3DSes, or from Wii Mii Channel. 3DS Miis can't be transfered to Wii Mii Channel, only from Wii.
Wii Mii Channel transfer protocol
The Wii beacons are similar to the usual multi-cart NDS beacons, except: beacon_type is zero, and payload size is 0x14. The payload data is just the Wii UCS-2 nickname, with some extra unused zero data. The usual multi-cast NDS protocol is used for sending the 3DS nick to the Wii. After many keep-alive frames, it eventually sends a bunch of frames, each containing the whole Mii. There's a 6-byte header, followed by Mii data. At the end of these frames like most NDS frames is the 0200 byte marker.
Mii QR Code format
3DS Mii QR is a standard 57x57 pixel Level 10 High ECC QR code with 'Mii' logo in center (refer to Denso-Wave Inc web site for QR Code format specifications). It contain 112 byte of binary data. 3DS seems to have fully implemented QR-code decoder, as it can interpret such Mii binary data being encoded even in the smallest possible for that data size Level 6 Low ECC QR code. The data itself is ciphered, algorithm is unknown yet.
|0x0||0x1||Magic Number (0x91)|
|0x4||0x4||High 4 octets of MAC address |
|0x18||0x1||Bit-mapped Sex, Birthday, ??|
|0x19||0x14||UTF-16 Mii Name|
|0x47||0x14||UTF-16 Author Name|