I do realy love the new feature in the doff settings, where I can set up heads of departments and the notorious first officer. But is this just a cosmetic feature or is there some impact on my ship performance in terms of maby sligtly better acc. on my torps, if my tactical officer is specialized in torpedoes etc.?
Hopefully as a feature in the future the department heads could also dictate where on the bridge view your crew sit - trivial as it is, it would be nice to see a helms man sit in the right spot of the Defiants bridge and having a option to set a uniform for each department would be cool too. so when you wonder around or call in a security team, or proc a nurse on the ground combat they are wearing a uniform of your choosing.
But to answer your question, no, duty officers are not assignable as department heads. Only your bridge officers can be placed in those positions. Check the "Department Heads" tab of your duty officer window.