Can you put disc brakes on road forks originally intended for calipers? I've decided I want a disc brake, at least on the front, which seems a good enough reason to go bike shopping, but am I overlooking the possibility of fitting a cable pull disc brake to my current bike?

I've been using a ten year old road bike as a commuter.  It has carbon front forks with road caliper/rim brakes and integrated Shimano road brake/gear levers.
To install disk brake, you need brake mount tabs on your fork/frame. Like this:


Original Image

And hub with mounts for brake rotor:


I've seen a few attempts to create an adaptor for fork/frame to install disk brakev tabs. But none were good enough for real life.

So your best route is to replace your fork and front wheel/hub.
But sometimes it is easier to get a whole new bike.