BMZ files are actually in most cases ZIP-compressed BMP images, which means you can create these simply by compressed the .bmp file and changing the extension from bmp to bmp.
As for rar to bmz conversion, this does not make much sense, unless there is a (.bmp) picture within the RAR archive. In that case you just repack the RAR to ZIP using WinRAR and change the extension to bmz.
Updated: October 16, 2023