Most Poser Figures are made in several separate segments (groups) in order to define the "Bones" for Poser's odd ball rigging system. Often the vertices don't line up along the seams of the groups to allow you to fuse the parts back into a single mesh that you could rig with Blender. Even if you could get the figure into a single mesh the polygon count would probably be so high it make the model almost unusable.