It has been quite a while since I have smoked any fish, but I have seen people smoke fish in a cardboard box so I would say yes you can use your old grill for a smoker. You will have to alter it a bit by eliminating some of the air leaks and rig up some kind of a vent control. I googled up a web site, and there were many. Here is one of them.
http://bbq.about.com/cs/fish/a/aa030400a.htm
When I smoke stuff..usually turkeys, I use some of the old branches from an apple tree we have, just soak them good for about a day.
good luck and save me a slab