They make adapters that shim a schrader valve hole down to presta valve hole size. Use one of those. Or, never mind and just use a presta valve in the existing hole. I've done so for over a decade with no failures. Presta/Schrader chucks are almost universal on floor pumps these days, and are available as replacements too - also an option. I've never seen or heard of something you screw onto a schrader valve to convert it to presta, and am having a hard time figuring how it would work in practice.