That particular sort of extension isn't possible yet - in theory it should work via Share, but in practice the only iOS app we've found where the Share feature reliably sends you a highlighted Chinese word is Safari, and since we already offer a dedicated web browser reader module it's hard to justify putting in the (rather massive) effort to create an App Extension just for that.
At the moment our plan is to wait until the iOS 9 beta launches in June and re-evaluate what to do then - they may add more extension types, or at least get the current ones to be integrated more consistently / cleanly (e.g. via a "Share" command from the text highlight bubble).