why would you ever do this?! They sell presta tubes for a reason...and they make rims drilled to presta for a reason, the 2 go together. If you want schrader for some reason on a road bike then by all means have at your rim with your drill, but don't complain when you either ruin it or it fails down the line.

Order whatever you need online, it's always cheaper and gives you a good reason to buy something else which would be too much at the shop.