If you need to convert your documents (.doc for example) to special OpenDocument Flat XML format (.fodt), your best option is to use some of the open source office productivity programs based on OpenDocument schema, for example LibreOffice or OpenOffice.org.
Convert Microsoft Word document (prior version 2007) to OpenDocument Flat XML document.
We have found 3 software records in our database eligible for .doc to .fodt file format conversion.
Apple macOS / Mac OS X software - convert doc to fodt on OS X
A full-featured set of office applications for Mac OS X
Convert from doc 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 doc to fodt. 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.
Multiplatform software - convert doc to fodt on any platform
Word processor and text editor from OpenOffice.org suite
Full-featured word processing and desktop publishing tool from LibreOffice suite