If you're using ScaleOut as a session provider, you can override the Page.PageStatePersister property to return a new instance of SessionPageStatePersister in the pages that need to use server-side ViewState.

We suggest being judicious when selecting the pages that will use server-side view state--you'll find many articles on the internet showing how to use ASP.NET page/control adapters to replace the viewstate persister on every page in your app via your web.config, but this is rarely a good idea since server-side view state will consume memory in your server farm... consider using it only on individual pages that will see a significant reduction in bandwidth usage.