Add panel element to the TabStripAdditionalContent property. This way it will be easier for the users to add content.

Declined: The existing API for the TabStripAdditionalContent property (of type Object) easily allows the users to customize and insert content depending on their requirements. For instance if you want to use data from ViewModel and list all of its Items you could use the following code:
<telerik:RadRibbonView.TabStripAdditionalContent>
	<ItemsControl ItemsSource="{}" >
		<ItemsControl.ItemsPanel>
			<ItemsPanelTemplate>
				<StackPanel Orientation="Horizontal" />
			</ItemsPanelTemplate>
		</ItemsControl.ItemsPanel>
	</ItemsControl>
</telerik:RadRibbonView.TabStripAdditionalContent>