TextMaker word processor should support Microsoft Office formats (Word, Excel, PowerPoint) just fine, so you can simply re-save .tmdx documents as .doc, or .doc files using the File ► Save as.. function. Just keep in mind that TextMaker only formatting might be exported to Word, especially since .doc files are from old version of Word.
Considering the limited support for TextMaker files, it is likely that this is the only way how to achieve tmdx to doc conversion.