Window capture always shows active video of the window being captured, it doesn't capture a single frame.
You could drop a scene with the windows capture in it into the sequencer and then export the take item to as an image file.
Or just Print Screen the windows, chop it up and export it as an image file in Paint.
Either way, you will have to create an image file of the window and then add it to an XPression material as a texture.
#XPression