Why would you ever want to do that? Not only will it not create a "better" sounding file, as has been pointed out, it may actually sound worse. Essentially the software is having to make an educated guess at filling in all the blanks, as it were. It's kind of like trying to up-rez a low resolution jpeg off the Internet-it can be done but you'll likely be introducing all sorts of distortions. You can certainly go the other direction and convert WAV to MP3 but can't imagine any reason to do what you've asked.