Hmm,... this didn't work for me. I just finished the tutorial, so I've got a 3-ship "Battle Task Group." My top naval officer is a Commodore (level 4). But the only option I had under "Staff Officers" was "Commander - Fleet Headquarters."
I tried selecting that, and yeah, it worked to assign her to Fleet Headquarters, apparently. But the Commander of that task group is still vacant. And I can't find any filter where that position shows up.
Well, I guess I'll just assign captains to the individual ships. I don't have anything for them to do, anyway - though I plan to have them start training as soon as I can remember how to give conditional orders (it doesn't seem to be on the Task Groups page, for some reason).