Daniel, there are different Audio file formats. There are differences between ones called Modules, and the standard MP3, OGG, WAV, etc. files. Modules are composed of predefined instruments, which means that each different instrument has a specific sound. MP3, OGG, and the rest are PCM audio, which means that it is composed of sound data, in other words, no predefined instruments or tunes. The MO3 format is used to compress the instrument sounds composed in the Modules. Because of this, you can't simply convert a WAV, OGG, or an MP3 into an MO3 file.
If you don't understand something, just ask please.