I don't remember if that's on the list of available formats (and I don't think it is), but to export anything out of Poser, you go to File -> Export..., and a window box pops up asking you what format. Now I suppose (assuming that VRML isn't one of the available formats) you could export Poser into some sort of intermediary program that can, for example, take an .obj file and convert that into VRML, but I have no idea how that would affect things like transparency.