My initial thought would be to use additional artboards. When the user taps on the button, navigate to a new artboard that has the additional content on it. Not the cleanest, but it should get the job done. If you are not familar with scroll groups, this also will probably be part of the solution.
Thanks. This was my only thought too. However, I think the status of components won't transfer between the artboards. So if the user has interacted a fair bit above the point where they make a decision that expands the webpage downward, those properties won't flow to the new artboard I don't think, so it breaks the experience. I could be wrong though... I'm new at Adobe Xd