I took a quick look and it seems that, while possible, 'Stepmania' on RetroPie would require a lot of work for a somewhat flawed result. You would need to compile it for the Pi, run it in the desktop environment to get experimental OpenGl support and then create a launch script to execute from Emulation Station. After that, integrating and mapping a dance mat would also be required. When it's all said and done, it looks like those who got it working previously saw poor performance. You can read up on it more yourself by just searching for "Stepmania Raspberry" in Google.