Are we talking front or rear? If front, you can almost certainly drill the fork crown to accept a normal caliper brake.

If rear, is there a bar above the wheel connecting the seat stays? If yes, you could drill it and install a caliper, if no you could maybe fabricate one that wouldn't be entirely unsafe. I'd get some 1/8" steel bar (at least), maybe 3/4" wide, use u-bolts to attach it to the seat stays (drill holes in the bar for the u-bolts), then drill and mount a caliper brake to that.