Are you having a problem with the front brake? If not why replace it?
If you ride in the wet a bunch, do long downhills with lots of braking, or remove the front wheel a bunch and have a hard time releasing the rims brakes, then a disk brake is worth it.
There are some kits that allow a rear brake on a bike without tabs. They usually require a special hub that has a carrier built into it and a torque arm going up to the brake boss. $$
If you want to make an upgrade to your Boulder that will be a good investment, buy a freehub/cassette rear wheel. The Boulder SE comes with a freewheel hub and this is the only weak point on the bike. If you ride it offroad and weight more than a hundred pounds, you will bend or break the axle. The freehub wheel is a worthy upgrade.