Tuna will cook very fast. I usually grill mine for about 3 minutes each side on a medium high temp. If you smoke it and want grill marks I think 1 minute each side should do. I marinate with a red wine vinegar and oil dressing with fresh pepper and a dash of lemon. MAN, I'M GETTING HUNGRY!!
Personally, i think a good tuna steak has a perfect light flavor, and i think smoking will put too heavy of a flavor on it. I just like it quick seared like above. Or for something different you could try Alton Brown's chimney method. I like it seared outside and rare inside with a side of wasabi and soy sauce.
I think the easiest way to do what you want is to use a smoker box on the grill, or even just some chunks tossed on the grill. Get the smoke going and the grill hot and then grill normally for 2-3 minutes each side (I like mine pretty rare). That will give it a nice kiss of smoke while not overcooking it.