Currently we do not have immediate plans to port this control for Windows Universal. Currently, I can suggest to switch to Popup.
I have logged this as feature request in our feedback portal where you can track its progress -http://feedback.telerik.com/Project/167/Feedback/Details/147195-port-radwindow-for-windows-universal
. If it become popular we will raise its priority.
P.S. I have updated your telerik points accordingly.
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