They can explode if you try that. Those ceramic magnets tend to be quite well balanced and if you drill a hole in them you disturb that balance which leads to a lot of tension in the magnet body. I've had this happen once to me and it was totally unexpected (to me...). Shrapnel from the magnet embedded in the works surface. I was wearing safety glasses. I was cutting the glue layer under the magnet with a diamond dust coated wire to get it loose from its carrier when the whole thing went 'bang', I must have accidentally damaged the magnet to cause this. What remained of it wasn't enough to determine where it went wrong.