No gamecube games can't be loaded through usb yet since gamecube mode doesn't have access to any of the Wii's other features so it can't use the usb ports for gc games. Possibly could be something done to make it happen in the future. Who knows. But for now it can't be done as far as I know.
Though if you're just talking about dumping the games and not playing them, I believe dvd dumper can do it for gc games too.