In-flight refuelling is a working feature. It is detailed in the new Manual BAT06-Modern Warfare.PDF, due to be released with the new B.A.T. Expansion this week. It is dependant on various mission conditions, so you need to know your way around FMB.
Tankers need to be above 3000m, in a suitable stable racetrack, and equipped where possible with extra fuel tanks. Player aircraft MUST have a fuel level below 100%, usually 40% is good, in order to be accepted for the hookup.
You can try it right away in my set of missions, the Siege of Khe Sanh, available under Single/UM (US Marines) which were added to Angels High.
Drogue Refuellers are very well covered, from the earliest Skyraider to the very modern Intruder.
Currently there are four principle Drogue Type Refuellers:
A1Skyraider
A4 Skyhawk
A6 Intruder (KA6D)
MiG-15KRTz
These hook up with suitable drogue type refuel equipped aircraft. Many combinations are possible. All of the USN tankers refuel these aircraft:
A4 Skyhawk family
A7 Corsair II Family (currently needs work adjusting position of probe hookup)
F8 Crusader Family
A6 Intruder family
FJ3M Fury
F100D/F (currently needs work adjusting position of probe hookup)
F105D/F
AV8B
F/A18C
MiG 15
Those aircraft with a permanent probe, like the A4, A6, AV8B can just hook up in proximity with the tanker using the Attach key (you have to map it). Those aircraft with a stowed probe, like the F105, F/A18 etc, must deploy the re-fuel probe first with the Deploy Fuel Probe key (you have to map it).
Its actually very easy to do, closure speed is very important and a HOTAS key assigned to "Attach/Detach Aircraft" makes it even easier. It does teach you a bit of finesse and is one of the more 'flightsim' features of the modern Il2/B.A.T. that I particularly enjoy. The up-coming Vietnam mission sets feature a lot of tanking up.
There is also a Boom Type refueller in B.A.T., originally modelled on the KB-26 Tanker hooking up with the F84G3. This models the earliest experiments during the Korean war. Other Boom type equipped aircraft are the A10, F4 and F16 family. They don't work with the current KB-26, which would be unrealistic anyway.
The future work on Boom Refuellers is very exciting, with suitable modern boom type tankers under development.
Look out for the updated manuals and features with the new expansion...