OK, so not impossible, but certainly difficult. I might PM whomever made those dragons later on to see if they'll let me use them, but I doubt it... Until then, on your imaginary forces work .
UPDATE: Hey, just a quick update, HannibalExMachina has let us use his sweet-looking units from Essos TW in this! As such, the release schedule just got a whole lot shorter. Until then, everybody go rep him!