Have you actually talked to her about more D/s in your relationship overall? This is not something you want to try to 'sneak in' on her. Think about how you envision it, write it down if that helps you clarify your thoughts, then talk to her. She may say "oh hell no", but she may say that she's willing to try. If so, start slowly. Maybe just have one "D/s day" per week or something at first.