Both .pmm and .pmd files come from MikuMikuDance, so if any kind of pmm to pmd conversion is doable, it would be done with this software. We have not found any specifics, it might just be related to some kind of export, but if you require something special, it might be perhaps to reach out to the MikuMikuDance online community.
Updated: July 24, 2023