Originally Posted by Sawdust

To clear things up.

Yes, you can resupply at a non-home port. Just get close to the port, hit ESC and choose "Dock at: <portname>"

You will enter your "office", and you can do some stuff, but not everything (e.g., you can't upgrade your U-boat). But, when you go back to sea you will have all torpedoes reloaded, all gun ammo reloaded, your U-boat will be fully repaired, and you will have been refueled.

When you do this, your new "mission" will be to go to your home port and the grid to which you are assigned will be "NULL"

This can be a convenient option if you are near a friendly port and your home port is far away.