You have to edit the Flotilla.cfg file. At the end of every year/flotilla block you will find a line that says: Renown00= 500 or something like that and there you must change it to the value that you want, let's say 2000.
You must change it for every year/flotilla that you want to start with the renown of your choice.
But notice that if you pull this value too much, somehow you won't get promoted, that means that you will stay as Leutenant Zu See until the end of the war. I think there is a limit of renown, anyway that's all it will happen. It will not cause you any problems.
CapZap