from LiquidIce’s Nintendo Wii Hacks:

MiiTransfer is a small application for reading and writing data to the internal memory of the wiimote. Using this app you can easily transfer mii’s from your wiimote to your pc, and back to your Wii.

This is exciting, because now Wii hackers can begin trading Mii’s using sites such as WiiModWii’s Mii Transfer Station:
http://www.wiimodwii.com/miis/

This was all made possible due to efforts by marcan and the author of WDML (WiiMote Data Management Library).

Here is how to transfer Mii’s from your Wii to your Wii-Mote to your computer. Once on your computer you can upload and download Mii’s from the internet, then transfer them back to your wii-mote and back into your Wii.

First you will need a bluetooth adapter and a Wii-mote.
The first release of MiiTransfer ONLY works with the Microsoft Bluetooth Stack. Yes this is the same MS Stack that will NOT work with GlovePie or many of the other wii-mote apps. If you are using another BT you need to go into device manager and update the driver. The next version of MiiTransfer should be more compatible with other Bluetooth Stacks.