There's no functionality for that. This means you have to implement that functionality. CB Auto Actions is a tool that needs to be configured for whatever usecase you want. In this case you need it to parse through every user that isn't approved and approve them. This can be pretty heavy as the Internal Users trigger that loops every user on the site does its filtering after it has grabbed every user. Your best option is a Code action with Method set to PHP where you query for every user that isn't approved so it's not grabbing your entire sites worth of yours.

I however don't recommend even doing this. It entirely defeats the point of admin registration approval. At that point why not just disable approval entirely?

I don't see the point of admin approval if you're going to have times where admin approval is ignored. Yes, that's possible but as I said above you have to code it as we have no built in, and no plans for, functionality for that. You can use CB AntiSpam to help protect against spam if that's the main concern.