You could create a routed command. Get the RadComboBox's ControlTemplate from the installation folder. For example on my computer it is here:
C:\Program Files (x86)\Telerik\RadControls for WPF Q2 2011\Themes\OfficeBlack\Themes\Office\Black\ComboBox.xaml
Add a button in the RadComboBox's ControlTemplate in its popup right below the Clear button. I believe there is a Grid there you could just add one more RowDefinition and put your button there. Add the routed command on the Button.
Now the good thing about routed commands is they bubble up the visual tree so you should be able to catch them in the RadComboBox, open a modal window, collect some data in a form and when the user is done editing add that item to the RadComboBox's ItemsSource and select it.
Does it make sense?
All the best,
the Telerik team
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