MoI is focused on modeling using NURBS objects, which are made up of smooth curves and surface patches. It's a significantly different way of structuring an object than a polygon mesh method, which is the kind of data that is contained in .obj, .3ds, .stl and .lwo files.

When you export from MoI to one of these polygon mesh formats, MoI will dice up its smooth surfaces into smaller polygon facets to create a polygon mesh. But it is not easy to go the reverse direction and take diced up polygons and create larger smooth surfaces back from them. It's kind of like how you can take a bunch of apples and smash them into applesauce, but it is hard to start with applesauce and go back to apples.

If you want to use a polygon mesh object as a kind of reference object to snap on to while drawing something new in MoI, check out this tool: Obj23dmWireframe converter which will allow you to convert all the edges of a polygon mesh .obj file into a line segments in a .3dm file which can then be loaded into MoI. That enables you to basically use the wireframe of the mesh in MoI as a guide.