By design, it was never intended that the player be able to do such (even before NE existed - believe it or not there was a "DK" faction but disabled in the vanilla code). The player can already manipulate the existing factions to great extent, and the insanely unbalanced supertroops the DK field make the player more or less invincible. Imagine the player as the DK marshall. Even with just a single Lord following you around, nothing's going to stand in your way.
Really, though, there is more to it - the DK weren't intended to be a faction that stayed through to the end. They were intended to show up and destroy, then eventually get beaten back and have the faction wiped out.
With the latest NE snapshot (to be released any day now), the DK's power has been curbed quite a bit, with their troops, tree promotions, skills & such being rebalanced. They'll be as powerful as they are now in the field against the AI, but far more beatable by the player. The goal of this long term is to add a lot more intrigue with the DK. For instance, I have some code ready to go that ensures when the DK arrive, they usurp a castle immediately, giving them a place to dump prisoners and regroup. Perhaps, down the road, when the DK are a proper challenge instead of just a brick wall, joining them will be re-added, and done properly (i.e. you do it through a quest chain, rather than simply rescuing DK lords until your rep with them isn't negative, which is not what the original devs intended).
