I'm creating a new mod and I didn't select country overhaul. Is there a way for me to edit the pre existing countries?

You will need to create a new country, then once you have done that if you import the relevant files for that country, you can edit whatever you need there.

States will not be imported, as they are not handled via the "country" part of modding - so if you don't want to changes the cores/the territory the nation controls, don't worry about assigning states, as the game already knows what that nation should have.

Thanks