[Evert Timberg]

Since the stall torque of the motor is constant, and that the potential energy stored in the spring is proportional to x^2, the motor could be moved such that the axis of rotation is closer to the piece you are pulling back.

At the stall point the lever (I'm assuming that lever attached to the motor / gearbox is being used to pull back something connected to the spring) is in equilibrium. The torque from the motor perfectly balances the moment caused by the force from the spring.

The moment caused by the force on the spring is equal to F*d where d is the distance from the point of rotation, by moving the motor closer to the spring (assuming you have the time to do this and it is feasible for your design) the spring could travel further before causing the motor to stall. This would increase the potential energy stored in the spring.