Yes, it can be done, however there are a number of considerations you need to be aware of.
The fork will be your main problem. If you want to go rigid, you could use a hybrid or cyclocross fork with an existing disc tab. Here you would need to be careful/aware of fork length and how that might affect front end/steering geometry. I'm assuming you have a disc tab on the rear, so the frame is not that old. In this case you should have 1/1/8 head tube and no big problem getting hold of a fork of this type. If you went disc, you could probably use a standard MTB fork, you would just need to be careful with clearance at the top of the tyre hitting the fork crown, if you could limit the travel this might work. You would be restricted in tyre size though.
I had a Cannondale with discs that I built some 700 C wheels for with road use and 'cross tyres in mind. That rocked, I'm sad that I had to sell it. The only issue there was fork crown clearance, I couldn't use any tyre bigger than 32mm, although there was obviously no bottoming problem with the headshok.
If you've got a fork with 26" wheel studs which you like (especially if it's rigid) you may be able to find/modify (file slightly longer slots) some cantis that will do the trick. Experiment with different types of cantis and see if you can make it fit, I've seen it work before. Be aware though that this will reduce braking power.
If you have a steel frame and fork, you could get a framebuilder to move the brake bosses. This wouldn't be cheap and would require re-painting afterwards, but if you're talking about getting disc brakes and new hubs and all it could be comparable.
Sam
