Propellant tanks are full at liftoff, so no, there's no way of adding more. And there's also no room to save weight either, considering that every bit of saved

Propellant tanks are full at liftoff, so no, there's no way of adding more. And there's also no room to save weight either, considering that every bit of saved mass is more payload capacity to orbit. Two features of the Falcon 9 Full-Thrust upgrade increased propellant limits, though: One was bigger tanks, but I think they said they're now at a height limit for various reasons (structural integrity of an empty rocket stage is usually abysmal, for example, and you want to land it upright and empty). The other was superchilling the propellants, which made them a bit denser, thereby fitting more.