I have the 18-55, and have used it behind the nx flat port as well as the nx dome port. I don't have the compact dome port. Port extensions aren't necessary for either of these ports. Personally, I prefer to use the lens behind the nx flat port.
I would recommend avoiding the compact dome port and sticking with either the flat or nx dome. I say this because the nx flat port works with the 60mm macro lens (one you'll want soon), and the nx dome port works alone with the 10.5 fisheye and 12-24mm (also lenses you'll want). Adding an extension to the nx flat port will also allow you to use the 105mm macro. So those two ports will give you the most leverage.
BTW, which version of the 18-55 do you have? From what I can see, the sea and sea gear I have works with the first version of the lens (with the wider diameter zoom ring). I don't see how it would grip the second version of the lens, which tapers to a smaller zoom ring.
BTW, I have outgrown my 18-55, so I would consider selling my zoom gear.
