The PWM frequency of the motor controller has to do with how fast it switches the motor on/off to control power. It has nothing to do with the kind of PWM used for RC hobby servo control.
You will need some kind of circuit that can receive RC hobby servo signals (from the RX) and turn it into the kind of signals that the control joystick outputs (typically a resistive divider/potentiometer.)
An Arduino and a few digital potentiometers could do it. I think there are ready-made controllers that can do it, too, from various robot-parts vendors.