Hola Anabelen, lo puedes hacer así:
<Window
...
xmlns:local="clr-namespace:TU_NAMESPACE_DEL_USER_CONTROL"
... >
<StackPanel Orientation="Horizontal" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<local:TuControl />
</StackPanel>
Espero te sea de utilidad.
Saludos cordiales.
Christian Amado
Software Engineer | MCPD: Windows Phone Developer | MCTS: Silverlight Developer
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