I have a typical recessed round plastic box for a ceiling light. I want to put a metal junction box on the outside of it and run surface-mounted metal conduit to two surface-mounted fixtures (in surface junction boxes).

I'm using the weatherized red-dot junction boxes (for aesthetics only - this is an indoor location). I assumed that I could just simply mount a metal red-dot round-box extension ring onto the plastic box and run metal conduit from there.

The extension ring I'm using is:

http://www.tnb.com/ps/fulltilt/index.cgi?part=DSE47BR

However, the extension ring box does not have a hole for a grounding screw. I assume it's meant to extend a metal box not a plastic one and grounds via the connection. However, what do I do in the case I want to extend a plastic box?

So what's the trick for transitioning from a plastic round fixture box to a surface mounted metal jbox?