You might be better off looking at Physics.OverlapSphere.

This function returns an array with all colliders touching or inside the sphere, and sounds pretty much ideally suited to your question.

You'd then want to loop through the list of returned colliders in order to apply the force to them, and since you're applying physics forces, all this code should go in your FixedUpdate() function. Something like this:

var colliders : Collider[] = Physics.OverlapSphere( transform.position, whateverRadius );

for(var hit in colliders) {
    hit.rigidbody.AddForce( whateverForce );
}

Yes, OnTriggerStay is one way to do it. You could also do OnTriggerEnter, set some kind of scalar on all the objects (like speed or whatever), and then set it back to the original value with an OnTriggerExit.

I know its probably sloppy, and lazier than doing Physics.Overlapsphere, but I sometimes like to attach a sphere collider to whatever I am putting trigger code on and hiding the mesh renderer in the inspector.