There is no converter. Although it's technically possible to "convert" mp3 to mo3, in most cases it's just plain pointless. Now, if you rendered your mp3 to a wav, you could load it as an instrument in a piece of software called tracker. You would continue by writing down a (musical) note which corresponds to the correct pitch of the wav, or actually two, one for the left audio channel, the other for the right. Then you would have to approximate the lenght and speed of this module you're making according to the length of the sample. Only then you'd have yourself a music module, which could be easily converted to mo3. This file is roughly the size of the wav you rendered earlier. To make it an mo3 and small again, the samples in the module are encoded to MP3, Ogg Vorbis or Lossless. Notice that mo3 files are decompressed fully before playing so in your case this means decoding the whole length of your music every time the music is loaded (ie. start of a level etc..)
