Yes you can replace the bottom-out bumper, but it requires disassembling the shock I believe.
I actually got mine changed to a softer bumper on my PEP rear, you can notice a pretty big difference when squeezing them. We'll see how it feels when riding
Tbone is right, you have to dissassemble the shock.
Any direction on the disassembly?
the easiest way to explain it would be for you to watch a youtube video on how to dissassemble. You will have to do a full dissassembly. Let the nitrogen out of the shock, remove the bladder from the reservoir, remove the dust seal and cap, remove the snap ring that holds the shaft assembly on, pull the shaft assembly out, grind the threads off of the top of the shaft (if it has never been revalved before), then remove the shaft nut, then the valve stack and piston, then you can remove and replace the bump stop. It's basically the last step in a complete revalve.