I have a large Brinkman charcoal with a crank handle for raising and lowering the charcoal and love it for grilling and smoking. I smoke using a offset method with a water pan over the fire and under the meat but the temp is hard ti control and not alot of space when I use 1/2 the grill for the water pan. My question is I was thinking about adding a side firebox to the grill and retro fitting it. Will this work? it's not set up for a box so I would need to modify by cutting and drilling.

Sure it will work. I'd get the smoke inlet down low as possible and put some sort of baffle (sheet metal "tent" or something) to help divert the smoke toward the middle of the smoker. At the least put a chimney on the end opposite the inlet so smoke goes across the grill.

For your firebox try to use something that's fairly tight and that allows you to have good control over the amount of air getting to the coals, which is the key to temp control.

Doug