Hey guys,
I'm at work now so I can't do a step-by-step or anything but it's definitely possible, just harder to figure out and less obvious than with the US side.
I started to make a list of which file to change or add for which type of boats and at which time of the campaign but it's not done yet. I also don't know if they'll be any side effects yet, like problems when getting a new model of conning tower etc.
This is my vanilla type IX in 1939:
I'll try to do a write up after I've finished researching and documenting the whole thing.