I have drilled out presta rims to take Schrader valves before using a hand-drill then some careful deburring of the hole, but the rim of course has to be wide enough to take that modification without being damaged. If the rim is so narrow that the drill size affects the sides of the rim and not just the inside then it may compromise the rims strength. Find an old Schrader rim and measure the hole by sticking drill bits into it until you find the max size that will fit, which will be the correct size, then see what it looks like when you fit the schank end of the drill inside the rim you want to modify and see if you can guess what material it will remove from the rim structure. If it will fit inside the rim without touching the sides, with just touching the bottom of the channel the tire fits into, then you may be okay to go ahead with the project. If I had a really nice set of wheels and was not experienced in this sort of work, I would certainly try to find a set of similar cheap or damaged wheels to practice on first. Grabbing a drill and experimenting on a cherished set of rims would not be smart.