Implementing Feedfoward from a Feedback Control System
Don't understand what you mean by measuring disturbance. Usually disturbance is some unwanted noise, that of course can't be measured. The feedforward loop path is a kind of benefical disturbance, but still a disturbance.
You may not close the loop with the feedforward, because it will change the system behaviour: a feedforward loop path is an open-loop control, while the controller operates in a closed loop. The injection of signal trough a feedforward loop path means a disturbance for the closed-loop, so if the controller is properly tuned so that will remain at safe margin from oscillation, then we can say that feedforward loop path has no influence on the stability. The HF excitation injected from FF loop path is rejected by closed-loop.
The FF is function of setpoint parameter,  MUST BE OPEN LOOP.