Attached Link: http://www.sparkyworld.co.uk/3d7own.htm

Yep it's possible - just like Megajax describes.  The process is sometimes called Chromakey in some video editors.

Set the poser background colour as "lime green" - render your poser movie and import it into your video editing software. In your video editor load your main clip - then import the poser clip as a 2nd 'track', find the chromakey option use the colour picking tool to select the green. Now play :)

Pinnacle studio is reasonably priced about 40UKP and can do this moderately OK. 

Sometimes if you anti-alias the render the video can have the fuzzies around the edges - espically with hair. If that happens try exporting the "video" from poser as a series of .PNG images and flood fill the white areas with the green -  warning it's bl**dy boring doing 400 frames this way :)

If you're working with Mini DV watch the poser render size and framerates (it's a b*gger to line them up right) 

The simplest way is to import the video clip as a video background within Poser. 
You can also aplly video as a texture (say to an plane) in Poser 5 and higher
Stick with .AVI as there video codec issues. 
  
Have a look at my tutorial (link above) for some fun ideas about using video within Poser.

Pinky - you left the lens cap of your mind on again.