the format is of mmpz is propitiatory, but save as mmp, and you have a std xml file.
Incidently, i recommend using mmp and not mmpz. If for some odd reason an error happens to the mmpz, then it is just lost. Unless the mmp is with no data, something kan always be rescued. Not that this is often at all, but it can happen, as with all other programs.
You save to mmp by just writing it the first time you save a project. All mmpz can be saved as mmp, and vis a vi.