LV won't add a hole in the strap - I've asked. But they did add an extra snap for me on a snap belt. It most likely has to do with how it will affect the overall look of the bag depending on how it was finished. For straps, the holes are punched in the leather prior to the stitching around the edges being put in, so adding a hole after would not result in the same look because the leather isn't the same as when the original holes were punched. Adding the snaps was probably a non-issue because there is no stitching on the leather, so it's the same as when the other snaps were added.
Not sure if this makes sense the way I typed it, but basically, if you absolutely want another hole, then you will need to find someone to do it and you should know that a) the hole will look different than the others and affect the strap, and b) LV will not repair any item that was altered by someone else.