I am a right-handed player who found this out by accident: if you set player preferences to left-hand mode, you can play the mirror image of a song even if it doesn't have a single-saber mode. Also, in Party Mode, you can practice single-saber songs with different usernames (I use "Rightie" and "Leftie") to keep track of each arm and strengthen the recessive one (in this case my left). These options have helped me mix things up a bit and not get stuck in an "exercise rut".