if I understand you, you want to add to a current ships armament, like AA guns, after you have started the game?
In my experiance, you can alter the tonnage, durability, armor, range, anything on the top of the editor page and it will correct when you reload the game. just click yes to any changes.
The guns, torps, radar, ect will not upodate. You can add a class update, make it take a day, and your new changes will take hold when you restart the game and then you spend a turn up gunning your ship.
Hope this helps.
I was trying to get sneaky by removing the guns and then upgrade to the same class but I guess the game is to smart for me.