If you want to do that, there are a few ways to accomplish it.
First and easiest, but most destructive... is to drill out the hole in the humbucker frame, to accept a wood screw. You alter the pickup. You'll need to either use springs, foam or shims under the pickup to set the elevation
The other way, is to inset escutcheons into the body, and use a machine thread screw. You'll still have to alter the pickup by drilling out the hole. The screw size to use would be 4-40 or 6-32