Hi Alano - the difficulty there is that OBJ files contain a very different kind of data than IGES files.

OBJ files usually contain polygon mesh data in them where the objects are made up of a big set of little tiny flat pieces. IGES files on the other hand usually contain CAD spline surfaces where the objects are made up of larger trimmed surface patches.

Since these are very different ways to structure data it can be complicated to transfer between these formats.

MoI can go from CAD data like IGES into polygon mesh data by dicing up all the big surfaces into little polygon pieces. But it's not so easy to go the reverse way and try to automatically reverse engineer large smooth surfaces from already diced up facets like you probably have in your OBJ file.

There are some specialized reverse engineering tools that can do it, but they are usually pretty expensive and usually used for converting scanned data into CAD surfaces.

If the OBJ file you are trying to convert only has a reasonably small number of polygons in it, it may be possible to do a type of conversion where every polygon facet becomes a trimmed planar surface in the CAD file. This is not a suitable way to transfer a high density polygon mesh file but it may work for simple files. To do that you could try getting the Rhino Mac beta and using the MeshToNurb command in there. But because there is a higher overhead for each individual surface in a CAD model structure this kind of conversion will not be suitable for dense organic models with some hundreds of thousands of polygons in it, so if that's the kind of thing you are trying to convert it will most likely not be possible without some special reverse engineering work.

The other thing you can do is if your OBJ file is set up to use sub-d smoothing there are some converters that can convert sub-d surfaces into NURBS surfaces, the main tools for doing that type of conversion are the T-Splines plug-in for Rhino, or the Sub-d NURBS plugin for Modo or Max:
http://www.tsplines.com/subdtonurbs
http://npowersoftware.com/subdtonurbsoverviewmodo.html

But just be aware that these types of files contain really different types of data in them (NURBS versus faceted polygons) and so it can be a complex issue to transfer between them, especially going in the direction of starting with polygons and trying to get a NURBS result.

- Michael