The crew tested the nosewheel steering power lever buttons on final approach to runway 04 at Sudbury, ON. Touchdown and deceleration were initially normal. As the aircraft speed approached 60 kt, the co-pilot, who was flying, engaged the nosewheel steering button. The aircraft veered sharply left and ran off the runway , stopping approx 4,000 ft from the threshold with the tail of the aircraft over the runway edge lights. 100 ft of the centreline was ploughed which was 50% bare and dry and 50% ice patches. There was a 2 ft high windrow on the west side of the runway which was located 30 ft from the lights for the entire length of the runway except at the intersection. Winds were from 340 deg at 10 kt. Both propellers and the left engine had to be replaced.