The last time I asked FFG about heroes joining command squads. I was pointed to this on page 23 of the Revised Core Rules.
A hero can only join a Command Squad if he has the Commander skill.
The answer was they had not to date produced a hero with the the “Commander” skill as of yet.
So I would say a Commissar could not join a command squad. But if attempt to bend you must, you should shoot FFG an e-mail to be sure.
This is from page 12 of Operation Zverograd.
A unit with the Commissar skill can join other units like a hero does. The Commissar
obeys all the rules of a hero joining a unit. However, the Commissar does not count as
a hero, therefore a hero may also join the same unit. Only one Commissar can join a
unit (although an individual Commissar can join a full unit of Commissars). A Commissar
cannot join an individual hero who has not joined a squad. A Commissar joins a unit for
the entire game and cannot leave the unit for any reason. If the unit is brought back into
play, the Commissar returns to play with the unit.
Finally look at the thread Commissars in general discussion. FFG sent m a second e-mail stating that Commissars are armed with knife and grenade.