Lol did you ever figure this out?

I didn't. Don't reckon there's a feature like that implemented. I fill the drum with a large jerry like a caveman.
Note, nobody confirmed either lack of the feature, nor its existence.

I wanted to do this also and was hoping there was a way. it makes sense that you'd be able to if you can fill up a vehicle. Should be a feature of this mod, and the game

What you can do is install fuel tanker vehicles, and fill them with gas instead from the pump.

I think your requests translate to (modding language-syntax) (and it's a summary of the requests)

the mod needs:
Context menu option /recipe for
-re-fueling/filling the barrels at the gas station pumps, vehicle tanks etc.
and
Context menu option /recipe for
-re-fueling using the barrels. Just like you would at the gas station pumps.
-- modded/vanilla vehicle , trailer-truck-tanks, etc.