You cannot share XAML markup between the different platforms. WPF uses a specific "dialect" of XAML and WinRT another one. So you need to keep your WPF user controls separated from your WinRT ones.
If you create a new "WPF User Control Library" project, you will be able to add WPF user controls to it as usual. But I am afraid you won't be able to use these user controls in your Windows Store App and vice versa.
Hope that helps.
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