Is it possible to add disc brakes to a bike with caliper brakes?
2 parts: front and rear.

For the front, getting a new fork with disc brake mounts is not that impractical if you are serious about disc brakes. You will have to get a new hub or wheel anyway. The front does most of your braking, so that may be all you need to upgrade. Get a mechanical brake that is compatible with your cantilever style brakes and you won't have to change your levers. I did this on an old touring bike of mine, and the improved braking in the front only was great and all I needed, even fully loaded on long descents.

For the rear, there are a few disc brake adapters on the market if you do a search. But they are heavy, flexy or both, compared to a frame mount. You can try them, of course, but I don't think they will give you the benefit you are looking for, esp. for the price of the adapter, new/updated wheel, brake, rotor, etc.