You can add a .XAML file to your project exactly as any other file, but prepare for the frustration: it won't add a WPF Window to your Forms application
. Do you understand that the nature of WPF application is very different; and it has almost
nothing to do with Windows API?
Interoperation between Forms and WPF is possible, but quite complex. Please see:
In principle, XAML is not directly related to WPF. This is just an XML schema which could be used for many different purposes. For example, you can use XAML as a vector image format. In this case, you can include it in your Forms (or any other) project as a resource and use for some graphical purposes. Somehow — you did not share with us your purposes. You could create some engine to work with XAML graphics or something.