Cutting holes in breaker boxes
The answer is Yes you can add holes to the back of the box with a hole saw. Hole saws and slug busters are used by Electricians regularly for this. A 7/8" hole saw will provide the correct size for a 1/2" conduit, 1-1/8" will provide the correct size for 3/4" conduit and 1-3/8" is the size for 1" conduit. I use carbide cutters because they are a bit faster and last longer but a standard bi-metal hole saw will work with oil or cutting fluid if cut at a slow speed (full speed / no oil will will over heat the cutter and the teeth will dull quickly). Make sure to provide a little space from the wall of the box so the lock nut will fit inside the box. I usually cut my holes at the top or bottom sides so the wires look neat and installed in a workman manor that code requires.