I don't know whether you actually can change the dose grid scaling value in Pinnacle. Instead, if you are familiar with Python, you could try to read the RT Dose file using pydicom, then modify the scaling according to your needs, and save the result as a new RT Dose file.
Reading the dose in Gy from the RT Dose file could look like this:
dcm = pydicom.read_file( rtDoseFileName )
doseGy = dcm.pixel_array * dcm.DoseGridScaling
where rtDoseFileName is your exported RT Dose file name and doseGy contains the dose matrix in Gy. Then you may modify the DoseGridScaling factor and pixel_array values according to your needs and write a new RT-Dose file.
I haven't tried this second part myself, but If that works as expected, you should be able to import the new RT-Dose file to 3D slicer.