I tried to set a resource convoy and there are only two ports you can choose, of course Liverpool isn't one of them. New Castle is an origin port but across the country.
1. Set AutoCreate Resource Convoys off (to manual).
2. Delete the Newcastle resource convoy. This will make all Island ports available as origin ports.
3. Choose Create New Resource Convoy.
4. Select Liverpool as Origin port and your largest German port w/land connection to Berlin as Destination port. This can include ports in occupied territory.
5. On your convoy screen scroll to the bottom and you will see your new convoy. Click on it to bring up the convoy details.
6. Assign convoys needed and click on Supply and Fuel boxes to include these as items shipped by the convoy. Rares, Metal, Energy and Oil are auto-checked.
7. Exit from the convoy screens.
The above allows the resource convoy to ship daily supply/fuel at a rate equal to the capacity of the Destination port (Resources have no limitation). These will automagically appear in the Capital Stockpile. AFIK you have to leave Resource convoys on manual creation from this point forward, but you could try to go back to auto and see if the AI "honors" your changes.