I know that this has been request by Zach Bracking before, but it was never answered by FormLabs:
We need a feature that would let us mirror/flip an object within PreForm.
I know I can mirror any object in an external application (like NetFabb, Meshmixer or whatever I used to model the original in), however, it would sometimes be more convenient to have that feature inside Preform. Also it would be very useful to be able to mirror an object has had the supports already generated. Opening up the mirrored parts separately, and re-orienting everything to match the original piece(s) and re-generate supports is not always easy.
Come on FormLabs, it can’t be that tough.
Besides, PreForm’s copyright notice states that it’s repair engine is by NetFabb. Netfabb has a mirror feature built right in. Why not use that code to implement the mirror within PreForm?