Converting a mp3 to a .mod/.xm/.sm file is NOT possible!!
This is something I ran into while trying to find out how to do this. It seems that any of these files are different from MP3-files.
Here is better info, and a place to find modules(.mod).
A way around this is to convert your mp3 to a .wav-file. However this is not a solution for me, but it could be for you.