As you know, when you create a Master Key, you select a combination of Lessons to give them when they register the key.
If you add new content (videos and/or downloads) to those existing Lessons after someone purchases, they'll automatically have access to them whenever they sign in.
If you know you're going to be adding new content over time, we suggest creating a special Lesson named something like "Master Key X New Content." Then add new stuff there.
If you already have a set of keys that have been purchased, you'll need to add the new content to one of the Lessons that was included when they bought the key.