You don't need to modify anything for traits, it's already an array in the file. Just add whatever traits you want, only need to make sure you type it exactly as the game wants, as i believe it's case sensitive on it. For example, here's the traits for one of my slaves:
"traits": ["Submissive", "Infertile", "Pervert", "Masochist", "Likes it rough", "Bisexual"],