Of course it can be refueled in space by a Starfarer! Interplay between ships is one of the key gameplay loops of Star Citizen and whilst the L Quantum Drive can get you places the slightly smaller tank will require you to do a little more forward planning. Since we're talking specifically about the quantum fuel tank, in UEE Navy fleet operations it's contemplated that the Hammerhead usually sticks pretty close to the big ships it's protecting.