I did exactly this; 5.5 gal sump/refugium on a 10 gal nano. It worked great; i used a maxijet 1200 as a return pump and drilled the 10 gal for a small overflow. If i were doing it again, I'd go external on the pump with the pan world 10PX; it's like a tiny iwaki. Or, I guess a mag 3 would work. Your problem, if your 10 gal is already up and running, is that you need to drill that tank if you want to add a sump/refugium underneath it.

You could try mounting the 5.5 gal so that the water levels in the 2 tanks are the same height, then run a u-tube over from one to the next, with a powerhead pumping water between the two. As long as the tube is submerged, it should hold a siphon. Definitely put the pump in the 5.5 gallon, so that if a siphon does break, you'll drain the refugium instead of the main nano.

Or, you can just buy another 10 gal for $8 or so, get it drilled (good luck finding someone to do that) and transfer the stuff in your nano to it.