Thanks for writing and for your question.
Currently there is not an out-of-the-box way to implement this behavior. We would not recommend doing this since it is against the general Windows Phone UX guidelines that Microsoft provide. When tapping on the Back button while a popup is opened the user would expect to cancel the pending action.
Can you please share with us some more context so that we can evaluate the case?
All the best,
the Telerik team
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