If you are talking about creating new physical motions (animations) that would serve the purpose of taunting, there is no possible way to do this (unless possibly through hacking the game). However, when playing online with Friends, you are able to create 4 custom taunting messages of up to a few words in length, which are displayed during the match by pressing up, down, left or right on the control pad (gamecube controller). This is done by adjusting your profile settings after connecting to Nintendo wi-fi. In your profile you can add or delete friends, and change your taunt messages and custom display picture. Aside from this, all characters have 3 taunt motions, with the exception of Wario who has 4 due to his ability to taunt while riding his motorbike.