I don't understand the problem with having additional countries in peaceful mode. Given peaceful merely implies no player based combat, I see no reason why you can't have additional countries acquired by taking over c3's. If you're concerned about building the country up and having someone take it, purchase WP.
Purchasing WP shouldn't be required if you are a Peace Level Player. Your countries would have to be profitable indeed to afford such a luxury. Though I am thinking about converting as my empire grows in wealth.