Similar questions about converting bikes that lack disc mounts have been asked, but I'm not sure if there's a duplicate question on converting wheels.
For a general wheel, the conversion process would mean that you need to remove the hub and spokes, and mate the rim to a disc hub. The original hub will lack any disc mounting tabs. It also wouldn't have been designed to withstand the torque transmitted by the braking action, so welding the appropriate disc mounts would not seem optimal, even if you had the skill and the ability to redo the aluminum shell's heat treatment. This doesn't seem economical, although it might be theoretically possible for someone with the engineering resources if they were absolutely dead set on doing this regardless of the economics.
In any case, this would also open up an ontological question: you would have reconstructed a key component of the wheel if you did the previous, and much more likely you would just have bought a new disc hub, so is it a conversion or basically a new wheel with an old rim? Also, I'm not sure how well a rim brake rim will withstand the forces incurred during disc braking, particularly the ones transmitted through the spokes. The rim bed might not be strong enough. I do not know this bit. In any case, if you had sentimental attachments to that rear wheel for some reason, this is a more subjective consideration that you might want to think about. I suppose you could post the question to the philosophy Stack Exchange site.
You specifically asked about a disc wheel of the sort used in time trials and triathlons. I am not that familiar with their methods of construction, but I am assuming that the disc bit is structural, and that the hub shell is bonded to carbon. So, in addition to the engineering difficulties I described earlier, it seems like you are now adding some challenges in reworking carbon. You would have to find a new hub shell. It would have to have the same dimensions as the previous one, and by that I mean all the physical aspects of the shell unless you want to enlarge the hole where it bonded to the carbon - and there could be implications for the wheel's structural integrity if you did this because you are cutting through fibers, and you do want to be careful if you do that. By the last part of the sentence, I basically mean that nobody without practical composites repair experience should even consider it, just in case there is a problem that leads to a structural failure.