Yes if it's already drilled for a 6mm brake bolt. If it doesn't have a flat face on both sides, you'll need a pair of special spacer washers with curved faces that correctly mate to the diameter of the bridge. These are especially critical if it isn't reinforced at the hole, since otherwise you won't be able to tighten the brake without collapsing the tube.
If it isn't drilled, it's probably still strong enough if it's at least 3/8" in diameter, and, again, if you use correct mating washers.
One question, why are you adding a second brake to the rear wheel? You'd be far better off adding the caliper brake to the front, since even a locked rear wheel can skid a great distance in a hard stop from speed. Like cars, the majority of the stopping power on bikes comes from the front brake.
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