There are at least three different products made for keeping the presta valver snug in a rim drilled for schrader.
1. A rubber grommet that snaps into the shrader hole and has a hole in it that fits a presta valve.
2. A metal collar that does the same thing.
3. A presta valve nut that has a shoulder on it that fits into a schrader hole. I believe the Schwalbe inner tubes come with this style nut as do a few other brands.
Brent