If you want the M/E NVFR rating you have to do the NVFR flight test again in the twin.
This is because you have to be able to demonstrate to the ATO your ability to identify and control the a/c under asymmetric conditions at night.
I think (though I'm not sure) you are also required to have at least 5 hours on type before you can do the test. I don't think the 5 hours need to be night hours. Though I note that it is a requirement for IFR to have 5 hours night.