Save as OBJ inside Sketchup
By this free plugin by TIG Export OBJ
http://forums.sketchucation.com/viewtopic.php?t=33448

then use this cool prog by Michael ;)
OBJ to 3dm (Don't use huge size like ZBrush OBJ file ;)
http://moi3d.com/download/Obj23dmWireframe.zip