Fuel Resupply?
 

Is it possible to resupply with fuel at places other than your home port?

yes you can go to any axis ports and be refueled

do you just park your sub in dock?

Get close to the port and hit ESC. Choose the option to "Dock at: xxxx", where xxxx is the name of the port.

You will end up in the office (just like when you are at your home port), but you won't be able to do certain things (such as upgrading your U-boat).

When you click on the wall map to start a new mission, you will be assigned to patrol grid "NULL", with orders to end up back at your home port. The good news is that you will have 100% fuel, U-boat will be repaired, and have a full load of torpedoes and other ammunition.

superb!

Or you can install Sansal's Milkcow mod. This adds resupply ships and uboats!

EDIT: (finally got his name right...)

Wow realy? do they move etc and have ai?

They don't move, they are stationary. Sansal made a map with their (historically accurate) positions