Don't get me wrong, I don't have Mastercam, so I cannot tell you exactly what to do. I'm just describing how I would deal with the limitations of certain file types, either losing the 2d data, or else the importing program does not translate the 2d into the incoming file.
So it is commonplace for points, lines and arcs to be handled by dxf format files. It is also commonplace for iges to handle surface and solid models. Depending on the nature of your MC translators, you would have to work within this limitation.
Thus, you can select only the points, lines and arcs, and cut and paste them into a seperate file, temporarily. This will be a dxf format file.
The iges, you can already use as is on import for your solid/surface data. The trick is that you might find a way to copy and paste (merge) the dxf data into your mastercam file, in the proper location.
Can you not extract edges of the solid/surfaces to create some of the contour entities that you are missing? As I said, these are just "general purpose" methods and may or may not be possible for you.
