There is a plugin for extra export formats at:

http://www.omnigroup.com/products/omnioutliner/extras/

but I don't think you should need that - File > Export > File Format includes Word 2007 (docx) on my system, which does export outline structure (visible in Word's View > Outline mode).

On the other hand, the output is 'manually' formatted, and doesn't respond to theme changes in Word. This is why I personally prefer to get a tab-indented Markdown text outline into the clipboard, and use this macro to generate a .docx outline which is accessible to global format changes with the MS Word Theme system.