So far as I know, the answer is no. You can't retrofit a coaster brake onto a regular freehub, 'cause the coaster brake mechanism is internal, so coaster hubs are made differently. But as Bike Nurse says, if you want a coaster brake wheel, you can probably get one for cheap. Shoot, if quality isn't an issue, buy a kid's one-speed from Salvation Army, steal the rear hub from that, get the rims and spokes of your choosing, and build the low-end coaster brake wheel of your dreams.