The Superstar will roll quite nicely. You need lots of throw on the ailerons (as much as you can get) & employ a little trick to help it. Don't wait until the plane has rolled inverted (or even to knife-edge) to input down elevator. Go to full throttle, raise the nose to ~ 20 deg above the horizon, then put in full aileron & push the stick all the way forward (full nose down) immediately after you put in the aileron (move the stick left or right, then forward as part of the same input). It will roll surprisingly axially. Be ready to return the stick to neutral & then add a touch of up elevator as it rolls upright. You will lose a bit of altitude, but not much. Stay at a safe height & try this in a vertical climb first to get a bit of practice.

You can also do an outside snap with the Superstar. Full throttle, nose up 20 deg, full aileron & rudder & full down elevator. Again, try this at a safe height & in a vertical climb the first few times.

The Superstar, like most flat-bottomed-airfoil trainers, is incapable of a decent inside snap -- the wing just won't stall in the roll.