Until this feature is implemented you could use the following custom implementation:
The RadRibbonBackstage is hosted inside an Adorner which is created by the RadRibbonView logic and we can not change it outside of the control. That is why we will need to remove the RadRibbonBackstage from that Adorner and place it inside a new custom Adorner which will be placed on top of the window.

In order to style the RadRibbonBackstage to look and behave as the one in MS Office2013 the following changes should be made:
- use the loaded event of the RadRibbonView to initialize fields which will be used later on: the RadRibbonView which will be used by to get the Backstage, the RootElement of the control which is used by the new Adorner.
- on the ApplicationMenuOpenStateChanged event of we are removing the Backstage from the old Adorner and we are creating a new Adorner in which we will host the RadRibbonBackstage.
- change the default RadRibbonBackstage style to have a button which will be used to close the backstage.
- expose a Command which will be bound to the new button and will be used to close the RadRibbonBackstage.

A sample project is attached in the feedback item.