You can link any slider or knob on your midi keyboard controller to basically any slider or knob in FL Studio. To do so right click on the knob in FL Studio and select “Link to Controller”.
Now slide the desired knob or fader on your midi controller and it will automatically be assigned to the fader or knob in FL Studio! It is also possible to link a midi controller knob to any knob on a third party VST plugin in FL Studio. Some VST plugins may not have the option to Link to Controller however there is another way you can do this: http://www.flstudio.org/index.php/automating-buttons-on-vst-plugins/