As I posted before, don't do it unless you are willing to risk damage to the gun. The new GUNS magazine has a very short article about shooting modern loads in old bp shotguns. The article cited some research by Winchester in which they compared loads. They used 12 g. trap loads, 3dram eq. 1 1/8 ounce 7 1/2 shot and the equivalant bp load - 3 dram (82 grain) of black powder, these both produced approx. 1200 fps. The pressure for the modern smokeless load was almost double that of the bp, 9600 psi to 4900 psi.
You should be able to find someone that can help you load bp shells.