EQ enhancements be made to an external/pre-existing MP3 file?

It's important to bear in mind that Audition - in common with pretty much all DAWs - cannot edit MP3 files directly. First they have to be decoded to the DAW's (uncompressed) working format - generally wav - and when you've done whatever modifications you want, they have to be re-encoded back to being an MP3 again. And this is a lossy process. With low bitrate MP3 files (think 128k) the quality loss will become apparent immediately. With higher bit rates, it won't be quite so obvious, but generally you get a bit of a plastic 'sheen' to them which really isn't very pleasant. So yes, you can do it - but what you gain on the swings, you're going to lose on the roundabouts.

Of the few direct (non-decoding) MP3 editors there are, none of them let you do very much. All you can do is a relatively crude level shift, and you can cut them up. But you can't EQ them, or anything llike that, I'm afraid.