Just treat the canvas like any other object you place on the grid and set it to span the entire grid

    <Canvas Grid.ColumnSpan="999" Grid.RowSpan="999">
        <Button Canvas.Left="200" Canvas.Top="100" Width="100" x:Name="btnMyButton" Click="btnMyButton_Click" Background="#77ffffff" BorderThickness="1" BorderBrush="#77000000">My Button</Button>
    </Canvas>