After you make the Midsurface feature in SolidWorks, you might have to use the Knit feature to join all the separate faces into one large surface body. If you do that, when you import into DesignModeler, you won't have to do anything. There will be 1 Part, 1 Body. But if you leave the face plates or skins for the honeycomb as separate surface bodies, you will want to Form New Part of the three bodies in DM to have the mesh connect across the edges touching the face surfaces by virtue of Shared Topology. You can assign the thickness in Mechanical.