You can do it with HTML tags, if you're allowed to use them and the site itself supports it, like this: <RUBY><RB>日本語</RB><RP> (</RP><RT>にほんご</RT><RP>) </RP></RUBY>
That yields 日本語 with にほんご as furigana.
I think you might be able to do it in Word, but I've never tried.