Hi Tom, it's probably going to be a while before this will be possible.
It's difficult because MoI is focused on working on spline curves and smooth surfaces, and STL files don't contain that kind of data. They instead contain a completely different style of 3D data that has surfaces defined by a large number of little flat triangle pieces and not any actual smooth curves or surfaces at all.
So it's kind of a mis-match for the type of data that MoI is focused on dealing with.
When you import an STL into Rhino, it will become a polygon mesh object inside of Rhino, which is a different style of object than a NURBS object. MoI will only read NURBS objects out of a .3dm file and not polygon mesh objects, that's why you were not able to read that into MoI.
If your STL file is relatively simple, it may be possible to run the meshtonurb Rhino command on it - this will take each polygon of the polygon mesh and turn it into a trimmed NURBS surface that are all joined together. This will not tend to work very well if you have a very large number of polygons to deal with though, it is just not how NURBS objects are designed to be structured - NURBS objects are designed to be made up of larger smooth surface sheets instead of zillions of little tiny flat facets.
Another option for now would be to save the STL out from Rhino to an OBJ file, and then you can run the OBJ to 3DM wireframe converter that is available here:
http://moi3d.com/wiki/Resources#Obj23dmWireframe_converter
That will create a .3dm file that has a line curve object for every edge of the polygon mesh data. That file with lines in it can then be read into MoI and used as a snapping guide.
But yeah in general it is a different structure of data than MoI is designed to work on...
- Michael
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