In OpticStudio’s Sequential Mode, we can model a mirror with a single surface of Thickness = 0. If this was converted to OpticsBuilder, it would be represented as an infinitely thin plane which is not realistic. In order to properly model a mirror in OpticsBuilder, you must be sure to apply a mirror substrate thickness before you convert the file. To do this, navigate to Surface Properties…Draw. There, you will be able to apply a thickness to the mirror which will make it into a 3D Compound Lens object that can be properly inserted into SolidWorks or Creo.

When the mirror substrate thickness is greater than 0, the converter will copy the sequential substrate thickness to the thickness of the Compound Lens object. If the mirror substrate shape is flat, as defined in this example, then the back of the mirror will be represented with a flat Standard Surface object. If the mirror substrate had been curved, then the back surface of the Compound Lens object would have been the same surface type as the front surface. 

For more information, check example #4 in the Knowledgebase article "Converting sequential surfaces to non-sequential objects."