Might anyone have experimented with video clips as elements or templates for background animations a la Avatar waterfall and skies design?
A little more information on what you wish to do would be helpful.
You can do it that way or you can apply it to a plane in the background. I think the later gives you more control. The first might be better for sky boxes though.
There are plenty of tools to make all of the adjustments you need. You want to be aware of the lighting in the clip and mimic it in the scene. It takes some work but you can get a really nice effect.