In ST3, I don't think you can "migrate" from single file to multitrack audio song. You would have to create each song from scratch.
 
You might want to consider waiting for ST4 which doesn't have different types of songs. You could then migrate your existing ST3 environment into ST4 (Full Backup of your ST3, restore into ST4) which would convert the songs. THEN you could go through the process of, song by song, replacing your existing pre-mixed stereo file backing track with individual stems. There would still be a fair bit of work, but at least the Song itself (title, artist, duration, tempo, lyrics, etc.) are preserved.
Happy to be corrected on any of the above if I'm mistaken, or others have better proposals!