The solution I see is to "rename" all the ranks below that one. For example, I want to add a Captain Rank for my Ground Batalions (US Ranks).
So I create a new Rank named "General of the Army" and it will be placed on top and will be the same of the direct below rank. The second rank is renamed from General of the Army to General. The next from General to Lieutenant General and so on...
When you arrive to the last rank, you will name Major to the new one you want, in my case.. captain.