Well, if you want it to have its own skin, I would suggest using Pack3d. You just have to make a copy of the Revenge/Royal Sovereign Class, rename it, and then open and clone the .dat file with Pack3d.
As for the skinning, I would suggest that you use the multi-skin .dat fiel created by UBOAT234. You may want to contact him to ask for the address of his fileshare page (where the file is located). A multi-skin capable .dat is much more efficient for adding new skins, even if you only want to use one.
(If you just want the USSR to have the Royal Sovereign in the meantime, you can just add the BBRevenge.cfg file in the roster to Russia's Sea folder and change the availability date.)