It would be nice if CrazyTalk could support using a movie file as the background (obviously, there might be some limits on output format).Right now a common workflow seems to be chroma keying the background, so that the file can be imported into a 3rd party video editor (Vegas, After Effects, etc.) for compositing. It isn't difficult to step through a Windows-media supported file and get the background image of each frame. This would allow easy use of royalty-free background animation sites like Pond5.
Microsoft has code in MSDN to obtain a bitmap-per frame from the formats Windows Media Foundation supports (which in turn, depending on installed software, covers most interesting movie types... e.g. Quicktime and DivX).