Harvey,
How about this:
WHEELSMITH Valve Adaptor - Converts Schraeder rim valve hole to presta. Alloy. Only 3 gr each. Pair. ADAPT ---$2.95
or this:
Rubber Valve Adaptor - Converts Schraeder rim valve hole to presta. Pair. ADAPTRUBBER ---$1.15
Both are available here:
http://www.gaerlan.com/
I've not tried them, but they sound like the two general approaches people have been suggesting.
Best Regards,
David -- I've got a wheelset with the same challenge -- White Burlington, VT