If it isn't in the bsa a substitute would have to be found since you can't take resources from one game and put them into another.
Sure you can. You open the Fallout 3 sound BSA with one of the BSA unpackers, extract sound/fx/ui/popup/ui_popup_experienceup.wav into the same path within the Data folder of New Vegas. That should override the sound in the New Vegas sound BSA, since it has the same path and name, just a different content.
You can't legally
post a mod containing this .wav file, since it's copyrighted, but there's nothing that prevents you from using it this way in the privacy of your own home.
Edit: I just tried it and it works perfectly. a quick "player.RewardXP 1" in the console and "Cha-Ching!"