I had a sunroof put in a Maxima I owned. I was shocked at how good it looked, even from the inside. However, they wired it wrong and it ended up causing me to get rid of the car eventually (well, one of the reasons I got rid of it).
I don't see how you could put an aftermarket sunroof in the Clarity. Those ridges and the recessed area would, I'm guessing, make it impossible. Also, on Honda's website, they make a point of saying how the Clarity's roof is laser-welded for greatly-improved body strength. Something tells me they didn't do that for nothing. Maybe something about the Clarity's construction means the Clarity's roof needs to be extra-strong.
All of that said, I'd love to see someone else try it and see how it turns out! I really want a sunroof. BTW, the one on my Maxima (a friend has it now) has never leaked.