There is already a game that combine strategic gameplay with tactical battles in real time, with seamless zoom from stategic view to battle. It's pretty cool, but somewhat exhausting to play once your empire gets large.
You can buy it directly from them, or through Gamersgate and Impulse (GameStop Now).
Longbow games are currently working on the sequel that will take place during Caesars conquests:
As for combining CK2 and M&B, I think it's more or less impossible to do it in a good way, without essentially rewriting both engines. I don't think I would enjoy battles using the M&B battle engine, unless it could handle at least an army of 10000 soldiers fighting on each side. Also, consider that CK as it is now, already takes a (relatively) long time to play. If this where implemented, it would take
at least twice as long, probably more. It would also rule out multiplayer. Not many players would like to sit idle while another players play a 30 min long tactical battle.
Finally this:
I think anyone that thinks adding a tactical game element to ck2 would be easy doesn't fully appreciate the challenges of software development. The content development alone for adding warband to ck2 would be enormous, figuring out how to represent 20000 soldier battles in warband, it would be alot of work for something that would inherently unbalance the game in favor of the human player. Integrating ck2 with a tactical battle resolution element would be exrremelly challenging to do well.
I would be happy enough if the TW series implemented the concepts of casus belli and bad boy status from the EU series.