Yes it should be possible.

In game, hold the ALT key
Add ons
GSX
Settings
And make sure the box "Always refuel progressively is ticked
And make sure the Detect custom aircraft system refueling is NOT ticked.

Call the fuel truck when you ready.
Is will ask how much you would like or if you want to custom the amount of fuel.

THat's how I do it. There could be a better solution. :)