If you render your scene on a blank background (such as a single character rendered against black), you can save it as a PDF (or certain other formats, but I use PDF), it will save the alpha channel as well. Then you go to the Channel tab, scroll to Alpha and click the little dotted circle to load Alpha channel as a selection, click back on the RGB view and copy and paste for a "lift" of the character without a background. Anyhoo, that's how I do it. If Poser can render each thing on a different layer, that's news to me.