Yes they can get them from other bees if they mingle during foraging,and from robbing out other infected colonys,or from drifting bees,especially drones which cover a large area, and are allowed into most colonys at certain times of the season.
wouldnt be hard to implement on a small scale. i imaging a big scale would be very hard. mabey it could be built into AS.
if it was used with the current drone culling method it could be very effective for very little disturbance. and to boot you get more drones which means your get better mated queens everybody happy.