Not yet, June or something its supposedly out.
It's highly unlikely that Anet are going to box dragons in expansions, the 4 of them we know about are in areas that wouldn't require an expansion to unlock. If anything expansions are going to be saved for new continents (Elona/Cantha) and the stories they will hold. It's likely that they will save the dragon stories & instances as massive events 6-9 months apart because lets face it a single dungeon isn't going to encourage people to buy new content.