Both .mmap and .mmas files come from MindManager brainstorming / mind mapping software, but only .mmap is actually mind map. The other .mmas files themes, particular looks that can be applied to a mind map to look certain way. As such mmap to mmas conversion only works in the sense of export of these themes from existing maps, if such thing is even doable, like when you make a new template.
When it doubt, it would be best to reach out to MindManager support or online community or perhaps even their support.
Updated: August 14, 2023