Most shops should carry a Presta Valve Rim Nut. In the Quality book it is part # TU6000 and as described as threads down valve stem to rim, plugs Schrader valve hole so that Presta valve is secure". Your Presta valve will have to have threads all the way down to the tube for the nut to work. We use a lot of these on mountain bikes that have been drilled for Schrader.
Mark, heading out for a spin, Poore