Add Window to Assembly

I have an existing project which compiled to a DLL. It has form based user controls and classes. Now I need to add a XAML 'Window' into the project. Is it possible? 

I am able to add XAML based user controls into this project but not a XAML Window as such. I need to Add this window as I do not want to host the XAML based user controls over a form, rather in a XAML window itself.

Please advice.

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