You can only contribute exercises to resources that belong to CK-12. Once you modify a chapter it becomes your own, and the exercise tab will no longer be exposed. If you wanted to add these types of exercises you could do so by embedding them into your pages.
To embed the exercises in your book click on the “Tools” icon shown with the exercise and grab the embed code. You can then use the embed code to embed the exercise, the same way you would embed a video.