I doubt that you can. To the best of my knowledge, you're stuck with just the pre-installed apps (and even those might stop working if Hitachi withdraws support for them, as manufacturers of smart TVs have a nasty habit of doing within just a few years of selling their sets).
For a definitive answer, phone Hitachi on 0845 604 0105 (0900-2000 Mon-Sat, 1000-1600 Sun, local call rate).