The twin-engine, turboprop airplane sustained substantial damage when it impacted the side of a terminal building while parking on the airport terminal's ramp. The pilot reported that he landed on the runway normally and taxied to the terminal ramp for parking. The pilot reported that he had brought the throttles into the Beta range (reverse pitch on the propellers) while taxiing to control the airplane's taxi speed. After he stopped on the ramp, the pilot put the throttles back into the Beta range and backed the airplane about four to six feet so that it would align with another airplane parked on the ramp. The pilot reported that he set the parking brake. He then positioned the throttles back to flight idle, and brought the propeller levers into the feather position. He reported that as he was putting the propellers into feather, the airplane started to taxi forward. He put the propeller levers full forward and put the throttles back into Beta. The aircraft continued to roll forward so he put the fuel mixture handles to the fuel shut off position. The pilot reported that the airplane struck the plate glass sided building going about 3-4 mph. The pilot reported that the brakes and Beta were functioning properly. The airport manager reported that runways and ramp surfaces had thin patches of loose snow, and that the braking action was good.