I am by no means a gunsmith, so I could be totally wrong. So take my information with a grain of salt.
Nitro powder is the old name for smokeless powder, so any modern shotgun is basically a "nitro" shotgun. That being the case, you must consider the major difference between black powder type guns and modern firearms.
Black powder and the synthetic powders that mimic black powder ignite very rapidly. What the gun manufacturer has to do because of this is make the actual compression chamber of the gun very strong. Modern powders all ignite at slower speeds. What this means is combustion of the powder takes place over much more of the lenght of the barrel.
Bottom line, black powder guns have to have much stronger compression chambers and don't need a real strong barrel. Modern firearms, on the other hand, must have stronger barrels and the requirements of the actual compression chamber is much lower.
Conclusion, most modern firearms are not designed to shoot black powder because of the higher chamber pressure, and likewise most black powder guns are not designed to handle the regular type modern gunpowder because they don't have as strong of barrels.
I, myself, would not even think about doing the conversion you are considering unless the actual model you are considering is one that gunsmiths regularly do that way.