It's a really interesting question. I did some reading and according to the Wiki, they didn't use it during WWII because they didn't see the need, especially with the range of the B-29s. It was only when jet aircraft became dominant that they needed a way to extend aircrafts' range. So for a Wunderwaffen DLC I can definitely see aerial refuelling technology -- as others said, the technology did exist during the period.
As for exactly how -- I suppose the most simple way would be to have Tanker air wings provide a range bonus to other air wings in the same air region, as long as the tankers flew Aerial Refuelling sorties. Or you could attach the Tankers to one or more specific air wings to give them a range increase.
I don't know how useful it'd be in a WWII setting, though. HoI4 aircraft already have pretty long range. Maybe tankers could give a bonus to air superiority and other stats as well, to simulate that planes are able to stay in the air for longer?