Best way depends on what video gear you have to work with.
If working with analog or a digital format that does not batch import stills with alpha channels (tif & png), then you can try rendering over a solid saturated color for a chromakey, but I've had pretty sloppy results with that & its tough to retain any background shadows.

Might be easier to go the other way and import the video into Poser as AVI background, then animate the composite and re-export for xfer back to video.