WELCOME ABOARD!
It's easy enough to edit the renown to give whatever you want, but I don't bother because I never 'buy' anything anyway.
Personally, I spend my entire patrol in the assigned grid for the simple reason that the game, even with GWX, gives many times the encounters that any historical u-boat ever had, and since I play for the historic feel rather than the tonnage I'm perfectly comfortable complaining about coming home empty once in awhile.
But that's just me.