In Render Options, set the background to black, then export/save your rendered image sequences to Photoshop or TIFF format; they have an alpha-channel. Uncompressed AVI has an alpha-channel also. Poser has a nasty tendency to reverse its alpha channel, so you might have to invert the alpha channel once you've loaded the render into your editing program. And turn off antialiasing, or you won't get a clean mask. You can antialias in postproduction, with a gaussian blur or something.