I’ve added outlets on interior walls but never exterior ones. For interior walls, you pull the old outlet out. Get a “old work shallow gang box”. Wire the new outlet in and bring wires off of it. Wire in the new outlet into its own old work shallow gang box. Before I attached my first time doing this, I read a few webpages on outlet wiring and watched a few YouTube videos.
NOW, the kicker I’m all of this is that you’re doing it on an exterior wall. That wall will be made of foam in the middle. I don’t know how the gang boxes would work there.