If a .doc or a similar document format that opens in LibreOffice and you have that suit, you could install the
BBCode extension for LibreOffice, that allows you to create a BBCode copy of the parts of the document you select. It's a bit barebones (it has to - BBCode itself is very basic and each forum adds their own extensions) but it does work.
A more roundabout way to get this done would be to export the document as HTML (most document processors can do this now) then run the generated HTML through any of a number of online BBCode converters that will appear when you use your favourite search engine.