You're on the right path. By default, the ICD group may or may not have all the extensions listed. If you don't list them, they won't appear, and you can tell that because when a call comes in on a line that appears on the new phone, it will be solid red.
Once you add it to the ICD, you can define how it rings/doesn't ring. Just add the new extension to the proper ICD and pick your state.
Carl