Yes you can you just need the homebrew channel and the homebrew browser. Once you got that you need to look for the app that is Mii extractor you need that and the Mii installer. Once you got that you need to go back to the homebrew channel load the Mii extractor once you done that your Mii's are now on your SD card.
To put them on anather Wii you need the Mii installer. Put your SD card in anather Wii that has the homebrew channel installed. load the Mii installer and it will take the Mii's right off your SD card. They are now on your Wii system. Thank You