Is there any way to project a shadow onto an invisible plane?

Here's what I'm trying to do:
I've created an "alien" for our graphics class.  I want to be able to put
the alien into a real image.  Ex. I may want to show the alien walking
behind me in a photograph.  The only problem I'm having is being able to
project shadows.  If I put a white box under the alien for the shadow to
fall on, the white box gets included in the alpha channel.

Has anyone tried this?  Any advice would be greatly appreciated.

Thanks
John

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