make a new nand backup of your Wii
restore the old nand backup
save your mii using official ninty methods or using savegame manager gx
restore newer nand backup
copy over your saved mii's
an alternative, which probably won't work at the moment is to use krafter's nand gamesave restore. Currently it only supports gamesave's, but he's said he's going to add mii support soon. Once he adds mii support, u can do the following instead:
extract old nand backup using giantpune's nandextract app (run's on your PC), then run krafter's nand gamesave restore to extract the Mii's from the extracted nand backup.
copy over your saved mii's using savegame manager gx
edit: looks like pune has an quicker method