It is true that a Zemax Part Designer file (a ZPO file) cannot be directly imported into the Professional edition of OpticStudio. However, this does not mean that you cannot use the part you designed. Instead, you first have to export your completed part as a CAD file. In order to do this, select The Part Designer Tab…File…Export As (shown below) to save your part as a STEP, IGES, or SAT type:

Once the file is saved, move back to the NSCE and import using the “CAD Part: STEP/IGES/SAT” object type. This will generate a static link with the designed part. Having a static link means that your part cannot change based on the optimization needs of your system. Only the Premium edition allows for an object’s parameters to be used as variables in an optimization in what’s known as a dynamic link which is only possible with a ZPO or SolidWorks/Creo/AutoDesk Inventor native part.