How to make a HwndSource transparent with Child window style

Hi,

I want to show some WPF controls above a Win32 window, so I use HwndSource to host the WPF control, and show it as a Child window of the Win32 window. Now I want to make the WPF control transparent, so that I can see the background of the Win32 window, so I set the UsesPerPixelOpacity to true on the HwndSource window, but I find  that this property only works if the HwndSource is a top-level window which means it must have the WS_POPUP instead of WS_CHILD, and this is not what I want. So is there any way to make the WPF control transparent and be a child of the Win32 window?

-Dominic