Not easily, though it's possible if you play around with P5's material settings. I've accomplished this in the past by setting the background and all material colours to white. stewer has a material trick where lit surfaces become transparent; there's a thread devoted to that over at RunTime DNA.

Poser 6 has a built-in shadow-catcher material option, in addition to a render-shadows-only feature. Good reasons to upgrade, if you do this sort of compositing often.