Hydraulic rim brakes can be retrofitted. They come with adaptors that allow them to mount on cantilever mounts. A set of magura HS33s is around £100 or so, so they aren't cheap. Also, be sure that your frame, forks and rims are strong enough. Hydraulics are several orders of magnitude more powerful, and because of the way they work there's no flex within them, so they flex the frame/fork (they need boosting to minimise this). There are recorded cases of snapped frames, forks and crushed rims due to magura rim brakes.