In order to convert mind maps from iThoughts format (.itmz) to MindManager format (.mmap), you have to use some tool with support of both of these file types, for example MindGenius. But it is likely that some other tool may be able to perform this conversion as well.
Convert iThoughtsX mind-map document to MindManager Mind Map file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert itmz to mmap on Windows
A business mind mapping software
Convert from itmz file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.