In and of itself, having a Cybersecurity Czar won't necessarily violate Posse Comitatus. Posse Comitatus is really a restriction on the use of our federal military within the U.S. borders for the purposes of law enforcement and/or pacification. If the Czar isn't able to order the military to take actions regarding domestic civilian networks and people, the Czar likely won't violate that particular act. The Czar could violate our civil rights and exceed the government's constituation mandate in many other ways, but without the military it probably won't trip over the Posse Comitatus Act of 1878.

I do find it horrifying that Congress wants to grant the Czar and our government the right to control or shut down civilian networks in the interest of "National Security" whenever they feel like declaring an "emergency", and I believe this ability would violate the 1st Amendment of the Costitution since it could easily curtail free speech.