Rigidbody.AddForceAtPosition & functions

First of all you should have split questions. But I'll try to answer both.

All physics calls you must do in FixedUpdate because that's where Unity recalculates everything physics related. You might not notice anything if calling physics methods in Update but usually behavior wil be wrong. If you see odd behavior check whether you have physics code in FixedUpdate.

And yes, you can set other parameters in FixedUpdate. But it runs with slower framerate than Update so parameters you update in FixedUpdate will not be seen immediately and their discreet values will span across several frames.

The second part. addForceAtPosition has the following signature: function AddForceAtPosition (force : Vector3, position : Vector3, mode : ForceMode = ForceMode.Force) : void. So you need to call it this way:

 hit.rigidbody.AddForceAtPosition(force, hit.point);

Where force is the force you want to apply.

UPDATE

To execute specific code in Update and do something in FixedUpdate you could use a coroutine like this:

 void Update() {
   // your code
   StartCoroutine(applyForce(hit, force));
 }
 
 IEnumerator applyForce(RaycastHit hit, Vector3 force) {
   yield return new WaitForFixedUpdate();
   hit.rigidbody.AddForceAtPosition(force, hit.point);
 }