I would never bother with plating or copper patterns on a step file, unnecessary excess data that you don't need in the 3D world for fit and form.
The only time you may ever need a copper pattern is if some really serious physical simulations are necessary. In the last few years since IDF transfer has been available, I have only done it twice, once for a space borne design and one for some sensors to go on very small birds. Both heavily funded projects.
For creating STEP files leave the copper off, it just makes the files large for no reason.