Hello,
One option would be to add a button to the Ribbon.TabPanelItems collection. That won't put the button in the title bar but it will be in the same row as the Ribbon's tabs, on the right side.
If you absolutely need a title bar button, check out the Metro-Style Window demo in the Themes section. That shows how a WindowChrome can be configured to show additional content there. You can get the WindowChrome that RibbonWindow uses by calling WindowChrome.GetChrome(window) after the RibbonWindow has been initialized. But note, that doing this might cause some layout problems with Ribbon like with contextual tab groups since I believe it's likely looking for a certain size of title bar button width there now.
