You do not need the DK Saga, just get the Flames of Vengeance expansion and continue your original character, or start a new one in FoV. The main difference between DKS and ED + FoV seems to be some balancing changes made to Ego Draconis that make the game easier and more streamlined. If you really want those changes there is likely to be a patch released later that turns ED + FoV into DKS if you want to.
