Hi Specktech,

For bringing a Rhino SubD object into MoI, use the ToNURBS command on it before exporting to a v5 3DM file:
https://docs.mcneel.com/rhino/7/help/en-us/commands/tonurbs.htm

Or also another route is use ExtractControlPolygon on the Rhino SubD object and then export the polygon mesh object to an .obj file and use the MoI subd converter under SubD > Convert > "From file".

- Michael