According to the "Letter from the Producer" on 2013-08-23,
Q: Could you provide us more information on when we will be able to develop add-ons and when official add-ons will be released?
A: We've just begun working on the environment that players can use to create add-ons. We've been receiving a lot of feedback and have managed to increase the completion rate of the base aspects, and once we develop the tool based on this we will be distributing it to players and letting you make your own. It'll be possible to make these add-ons in approximately 6-8 months after launch.
That being said, I would love a more recent estimate, or an idea of what we can access, or what could be changeable.