Easily. Go to Versiontracker.com and download Amadeus, Sound Studio, or any other OSX native sound editor.
Then comes the fun part. All of these types of programs should be able to open an MP3, but they only handle up to two channels of audio in a file, and the chances are that your MP3 is stereo, which takes up two already. So open up the song in the audio editor you downloaded and save it as a mono file. Then open it up and add the vocals by adding a new track, or copying the audio from the mono file into a new stereo one.
If you want to not have to do that, then the easiest way would be to download ProTools Free and run it in OS 9. Then you can have up to 8 tracks, and there is none of the mixing down mentioned above.
Good luck with your recording.