Use the DocBook XSL-FO stylesheet to convert your file to XSL-FO, and then use XMLmind's XFC product to convert the FO to RTF, which Microsoft can load. XMLmind's XFC is an XML conversion engine, with one output being RTF. It comes in a free Personal edition, and a Professional edition that is not free. See the XMLmind website for more information.

If you are willing to author your DocBook using a subset of its elements, then you can use the stylesheets that are included in the roundtrip directory in the DocBook XSL distribution (these stylesheets were created by Steve Ball and were in the wordml directory prior to version 1.72 of the stylesheets). These stylesheets can convert a subset of DocBook elements to WordML, which is an XML vocabulary that Word understands. You can also export from Word to WordML, and convert that to Docbook using other stylesheets in the same directory. See the file named supported.xml in the roundtrip directory for a list of supported DocBook elements. See Steve Ball's website http://www.explain.com.au/oss/docbook/ for documentation.

Jfor is an XSL-FO to RTF converter that is maintained on SourceForge, but it appears to no longer be an active project.