You can use telerik's ContainerBinding and bind your view model property to IsExpanded property of the RadExpander. This link
gives more information on how to use container bindings. If you meant something else when you said "according to your prism environment" could you please elaborate more on the situation at hand and what you want to achieve. I'd be glad to further assist you.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.