You can install presta tubes in shrader holes, but much better to install a pair of Wheel Master alloy stem savers.. The wider 700C rims can be drilled to accomodate shrader valve stems without weakening the rim, so that's what the rim manufacturers do. Here's what they look like...
http://cgi.ebay.com/BIKE-BICYCLE-PRE...d=p3286.c0.m14
I installed these but first modified them with a small hand file. I shaped the lip to conform to the inside curve of the rim. They install before lining the inside of the rim with cloth rim tape.