Your question is not very clear. WHAT format is set in stone? If you are referring to the .mmm extension, that is the project file, not an audio file. To produce an audio file, you must EXPORT the project.
You can import any WAV file into MMM. In MMM, you can do basic editing, such as length, volume, pitch, BPM, fade-in/out as well as equalization and adding effects.
To do in-depth editing to a WAV file, you will need to use the Music Editor that usually accompanies the MMM software.