It looks from your picture like you have the mk2 frame with swapouts and disc mounts.
Adding a disc caliper to the frame then is easy – it will just bolt straight on. You will need new forks though – with a disc caliper mount.
You will need to change the wheels though (or at the very least the hubs in your current ones, which prob. wont be very cost effective) to disc-compatible ones – so the hubs have the mounting bolt holes for the discs.
It could end up costing quite a bit!
I’d investigate mini-v’s first – they are infinitely better than canti brakes in my experience, and will work with your existing set up. They should only cost £30 for both front and rear.