Hello peabody,
When you add a Sub it has to be German or American.
So you can only overwrite the German campaign.
But this shouldn't be a problem
, cause there are nor big signs who say you are a GERMAN SUB...
e.g. you can rename: Crewnames, Subname, upgrade names, Harbor names, Menu names, flotilla names and so on
Praticaly you can make every unit playable for every country.