Then there are Kelly Take-Offs. You can adapt DT shifters to sit on stalks that extend from the side/Botom of your original Brake Levers. You will still need to find a DT cable stop or clamp band that you adapt. There are adapters that don't have cable adjuster barrels, but your Rear derailleur probably has an adjuster built in. See:
Kelly TakeOff - Kelly Bike Company : Kelly Bike Company