Front and rear brakes have very different issues.

For the rear wheel, the solution is relatively simple: you could either have disc tabs brazed on or add a bar that allows one to usea disc brake on existing tabs (I've seen it on the web but can't remember the link). Brazing tabs is much nicer, but you need brazing abilities (or know someone who has) and factor in some repainting.

For the front wheel, you'll need a stronger fork to witstand the added torque that comes from braking, and the fork will need to have lawyers lips precisely for the same reason. If the old bike has a threaded headset, you'll be challenged to find a fork which is at the same time disc-compatible and threaded-headset-compatible.