Blender has a shrinkwrap capability that will formfit a new mesh on top of another mesh.  I haven't used blender, so haven't much advice there.  This should create a new mesh without all the internal geometry of bricks that aren't visible to the external view of the lego kit.  In other words, it should get rid of the hidden interconnecting meshes of the bricks and only leave the external view (assuming that you're not trying to make the mesh in a fashion that allows pulling apart the individual bricks).

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon