I have a 700c wheelset with a Sturmey Archer S2C rear hub, which is coaster brake. It's fun on a bike dedicated to it, but there's more to it than just adding a coaster brake. Like, you lose all your gears and are just riding a singlespeed, and you better find a magic ratio if it doesn't have either horizontal dropouts or tension adjusting mechanisms, because a tensioner won't work with a coaster brake.

More trouble than it's worth -- get a second set of wheels set up with road tires and a more road friendly cassette if you just don't want to swap your tires over for road rides.