As far as we know, the easiest way how to convert various mind-maps would be with some exchange format like the OPML. Since version 8 Xmind supports for both import of export of OPML files, so it might be now possible to use this format as a means to transfer the mind-maps between iThoughts and Xmind apps. Direct itmz to xmind conversion remains troublesome.
Open ITMZ file Open XMIND file