One way to do this is to make the object it is orbiting around more massive. The more mass the orbital parent has, the faster an object has to move around it to maintain its orbit. I don't think there is a way to increase solely an object's speed and still have it occupy the same orbit. However, if you make the orbital parent more massive, orbital speeds will increase for every object in the system.
Alternatively, if you don't mind the outer planet's orbit being elliptical, you can increase the eccentricity. It will have the greatest speed when it is closest to the star in an eccentric orbit.
But otherwise, there's not really a way to break physics to that extent yet.