Why didn't Lelouch incorporated Black Knights in imperial army using the fact that he is Zero after he became the king of Britannia? After Lelouch became the ruler of Britannia, he could have told them that it was part of his plan and used them against, but he decided to just recklessly attack.

Why didn't he incorporated Black Knights in imperial army using the fact that he is Zero after he became king of Britannia?
Well, there are 2 reasons.


Leaders of Black Knights felt betrayed after learning his Geass power and potential usage on them. 
Also after trying to kill Lelouch they announced Zero died in battle thus eliminating his chance to announce her Zero identity. Even if they didn't revealing himself as Zero would be a bad move.