I may be wrong but devil are a species, they are not damned souls. You can create, since it is D&D, some kind of pseudo-devil that was once a very evil human that brought destruction etc etc and then it was granted some important place in hell. You can work with that, somewhat, but even then it would be hard to find redemption, I mean hell is by definition evil and working with the forces of hell corrupts, it would be hard to make the redemption arc work without making seem forced or the arch-devils incompetent enough to have granted such a gift to a mortal soul.