I know of a few different adapters made to protect presta valve tubes when used in Schrader drilled rims.
1.) A company named Pyramid makes these simple rubber grommets which fit into Schrader drilled rims:
http://www.bikepartsusa.com/
2.) Wheelsmith makes flanged aluminum stacks which extend out after resting on the inside of a rim:
http://www.bikeparts.com/
3.) Panaracer makes a knurled Presta threaded nut with a short extension to nest into a Schrader rim opening:
http://harriscyclery.net/
I'm not clear about how these last adapters work. If screwed down on the outside of a rim it would be only useful when pressing the valve stem inward with a pump, so you would need a washer on the inside of the rim to keep the stem from pushing an inflated tube outward against the rim wall.
If screwed on facing outward before inserting a tube, you could add a standard nut on the outside which would then tighten the two nuts against the rim. This would make the most securely fitted valve stem ever.
BOB HANSON, ALBUQUERQUE, NEW MEXICO, USA