It seems that the only way to produce a .itmz file from .xmind files would be to use the iThoughtsX brainstorming tool for OS X and iOS. Maybe some other brainstorming tools allows for this transformation as well, but we sadly do not know about any.
Convert XMind workbook to iThoughtsX mind-map document.
We found 2 software records in the database related to .xmind to .itmz file format conversion.
Apple macOS / Mac OS X software - convert xmind to itmz on OS X
A mind mapping software for Mac
Convert from xmind file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned xmind to itmz. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple iOS (iPhone, iPad, iPod Touch devices) software - convert xmind to itmz on iOS
A mind mapping app for iOS