The stiffness is set by the valve in the rack that translates steering wheel torque to assist pressure. It could be that the valve is adjustable, but the only one I ever got into (ages ago, and not in a rack and pinion system) was set by a single spring. To complicate the situation the same valve is responsible for centering the steering force - if the valve is off center the steering will pull - so adjustment is a bit of a trick. Because of this I doubt adjustment is an option.