Exporting .step to Solidworks

Hello everyone!

I have a construction that consists of different parts in Solidworks 2018. I wanted to use topology optimization to reduce the mass, so I turned to Altair Inspire. After optimizing the geometry I need to export it and replace the original part with the optimized.
So I use Fit PolyNURBS command to create a single part out of multiple (optimized geometry + regions that were not supposed to be optimized). Then I select PolyNURBS Part 1 in the list of parts on the left side of the screen and go "File-Save Selected".

The problem is that when I open .step file in Solidworks it turns out to be an assemblage, not a single part. 

Do I do something wrong? Is there any solution? How can I export a single solid part to use it for further construction development in Solidworks?

Thank you in advance!