Hey, my skinned meshes on 3ds max uses biped skeleton as Editable Mesh (have nothing to do about it), this way, when I export to FBX and use AssetImporter from Urho3D, the skeleton is considered a normal mesh, and render this normally.
I usually export skinned meshes in x format from max and had the same issue.IDK if it helps but try to hide all your biped parts before export.It works with x