Hey – your hub will either be disc specific or not – you won’t find any way to add an adapter of any sort (frankly – when it comes to brakes, I wouldn’t really trust the concept). If you want to go disk, you’ll need to buy a hub, or whole new rear wheelset. If you decide to go just hub and re-build the wheel, make sure you are looking at all the right parameters for what you have in stock – spoke count, disc bolts, etc. (obviously gear cassette size won’t be an issue…)