As already said, yes, they can be reloaded, but it isn't recomended, and is in fact dfiscouraged against by component manufacturer's. I wouldn't personally ever put a steel casing through my dies, ever! There are characteristic variances in just about every aspect imaginable that would likely make it a process of developing your own data due to the way steel acts when resized, and then fired. It doesn't spring back in the same manner as does brass, and therefore could produce a significant increase in pressures using data intended for brass, which all data is tested and rated for brass.
As for nickel plated brass, I've been loading it for many years and have never had any problems. Some say they will not last as long as yellow brass, but I get pretty much just as many cycles from them as from standard brass.
GS