The first computer based Mii editors have been released. One is on online flash based editor, while the other one is a Windows application. These editor programs will allow you to edit Mii's from binary files on your computer. Then you can use an application such as MiiTransfer or MiiMegaMall to copy those Mii's to your Wii.
Both of these tools are in their beta/alpha stages, so this is for testing purposes only. For more information on the Mii file structure check out the WiiBrew Wiki.