I believe that the PV Python interpreter will pick up directories in your PYTHONPATH and load those modules (as long as the version of Python matches). So, you should be able to install opencv-python using whatever method is easiest for you, set the PYTHONPATH to point to the installation, and then run ParaView (or pvpython).
On a side note, if you want an easier method of installing python modules, you might want to look into anaconda. It make installing Python and distributions lots easier, particularly on Windows.