I don't think people realize just how many endings this game has. http://img15.imageshack.us/img15/912/nvendings.jpg Next to each faction or person I've marked how many endings they have. As you can see, for most factions and followers, there are multiple endings. Can you imagine the time and effort it would take Obsidian to create content for every single one of those endings, even though each player will only see a small fraction of that content? It would be a massive undertaking for little payoff. Obsidian not only doesn't want to do a Broken Steel-esque DLC, they can't, unless such a massive challenge that would ultimately lessen the impact of the endings they've created sounds appealing to them. No, it's not technically "impossible," it's just really really really really unlikely to happen.
Broken Steel was possible because Fallout 3 only really had 1 big difference in it's two general endings, which was that you either put FEV in the water or you didn't. And if you did, in Broken Steel nothings changed except you can't drink much of the water. NPCs can, but you can't or you'll die. Before Broken Steel, the ending made it sound as if there would be a huge amount of people dying from the water, but it was implemented so poorly in Broken Steel that it appeared as if it made no real difference to the world around you. I can't imagine it would turn out any better for New Vegas' endings.