Suzaku is Euphie's knight still, even if she's dead.
He's only Nunally's assistant.
As for the OP's theory, I could see it happening, but that would really put kallen in an odd spot. She would not want to fight the black knights (and probably wouldn't have to since lelouch would not attack nunally), but she would not want to idly sit around the capitol building doing nothing. She probably feels no animosity towards Nunally, seeing as how she's never treated her bad and Lelouch still cares for her, but I dont know how Kallen would react to the proposed knight-hood.