I think it would be great if duolingo implemented this feature, it would make the learning of kanji words much easier
It would certainly be nice, but I don't think DuoLingo's software supports the addition of furigana.
It is unlikely that this will be added to the course any time soon.
Note that if Duolingo's Tinycards was to support HTML, we could just use <ruby> and <rt> tags. See also: