How can I export all my data into a single file, for instance a Microsoft Word file ?

You can build a WIT website where all ideas are built in a unique HTML page :
    - open the structure template (expert mode),
    - choose the "Page" tab,
    - and check the option "same level ideas grouped on a single page from
level 1"
    - build your website
    - open the single HTML file with Word
    - save it as a Word document.

Note : you may also remove additional navigation items such as child links or the hierarchical position from the structure template to get a more compact file. You may also remove all bullets and images.