IMO, not modify butt end is best. I always do make own screw to adapt my cue extension. Nowadays, most of custom cue makers depends on making cue extension for their custom cue using extension kit (screw bumper, and pin male) from China. Even they have to change their original design methodology of making cue to adapt cue extension. Sure they will have to calculate balance point if they could, but most of them not achieve success.
If you're willing to modify, just bore the butt, do plug and do thread. Plug it with hard material for long lasting.
Just my 2 cents
