You can resupply at:-
Your home base.
A friendly port.
A resupply ships (I am guessing this is the one you are looking for info on)
Resupply ships do work in the stock campaign, but OHII make better use of them. It also uses them more often and will expand on this with the research that Gap has done.
Here we see a resupply ship in a Spanish port where you would not nermally be allowed to resupply.
The stock game also has a bug where you can only re-supply once per patrol (that is if a supply ship is provided for that patrol and you are within range). So asking your radio man again and again will not work.
You will get it first time if there is one.
You can use TDWs patcher to fix this supply bug and then you can request as often as you like.