The most simple solution for a clean, controlled enlargement of the hole is to create a punch from tubular steel. You would need to sharpen the end of the punch (the sharper the better).
Then you will need to support the bumper directly under the hole using a block of wood. This should be sitting on concrete, or a very solid floor. Position the punch concentric to the existing hole, then whack it with a hammer. The best result will come from one firm hit with the hammer, not multiple attempts.
If you don't have access to a suitably sized steel tube, you may find some other utensil or thing laying around the house - for example, somebody's good pen. Also note that it's possible that the hole will end up a slightly smaller diameter than the tubular punch, so you may want to compensate for this 🙂
Actually - I don't own a bumper myself, nor have I seen one in person, so I don't know how hard the plastic/rubber is. You will have to use your judgement whether the above method is suitable (if the material is too hard if may result in cracking or splitting). Good luck.