updating prefab without changing instances, no code

Let's say I have a prefab of a GardenGnome. In it I have added a component with string gnomeName field where I will give names to instances. Then I place two instances in the scene. One of them gets gnomeName set to Bob the other one Teddy. Now I want to add a component to the GardenGnome prefab, say a SphereCollider. How do I do it without damaging instances in the scene?

If I add SphereCollider to Bob's instance and hit apply, Teddy's name will be overriden and set to Bob.

I can't add SphereCollider to the prefab from the assets window as there is no such option.

How do I do it without adding SphereCollider to instances on scene, by hand, one by one so that my instances only update default values of fields but don't override the data that was set manually on instance in scene?