ONE CANNOT SPOT REDUCE FAT.
"Toning" as you're using it is impossible. "Toning" is nothing more than the process of shedding bodyfat from the entire body, to the extent that the existing muscle underneath becomes more defined because less fat is covering it.
Thus there is no real way to exercise for toning (you may have heard low weight and high reps help 'tone' a muscle...that's a myth).
Now, to your question: stair stepper is brutal, and works the lower body pretty hard. It is indeed possible to build some muscle in the glutes on this activity, but its not going to be significant. any increase in size would also result in an increase in firmness, and a nicer curve to the rump.
stairstepper isn't gonna give you a booty like J-Lo.