Why not ? Make an army level HQ under the Theatre HQ, call it Admiralty, and you have your Admiralty. The Admiralty is based on land, so it is a land unit. Army and Navy, although kind of indipendent, still need to coordinate each other, hence they need a higher HQ in common (presumably Theatre HQ). Makes sense.
Only problem I have w/ the command structure right now is that your "Admiralty" will have a 5 Star General instead of Admiral in charge.
c'mon.... MacArthur / Von Kluge in charge of the US Navy / Kriegsmarine?:rofl: