No, they cannot. From http://wiki.infinitythegame.com/en/Fireteam_Integrity If the trooper activates or is in a state that allows it to be substituted by a Marker (Camouflage, TO, Impersonation, Holoecho...), unless a Special Skill, piece of Equipment or some Scenario Rule indicates otherwise.
Just to be clear, the hacker can declare the Cybermask skill as part of the Fireteam, but doing so will cause it to be kicked out of the Fireteam due to the Fireteam Integrity rules. You don’t get a situation where you’re running around with an Impersonation marker in a fireteam. And since the hacker needs to be the team leader to perform the skill, you lose the fireteam when it gets kicked out.
Short version: Only marker state allowed inside a Fireteam: Holoprojector N3 (currently only in Patroclus' profile, and can only Duo with Achilles V2).
