it is perfectly safe to use stainless steel wool on a Vitreous enamel. I presume that the cast iron skillet has a Vitreous enamel finish.
The reason is simple:
Vitreous enamel is essentially a type of ceramic and has a hardness of around 5-6 on the Mohs scale. Steel has around 4-5 on the same scale. This means that generally, you can not damage it with the steel wool.
Also no not confuse the "grit" of the steel wool with its hardness :P
The only thing that will happen is that the enamel will get very small scratches and that things will stick a bit better in them. You can get rid of most of them with a good polishing compound if they disturb you to much.
Personally I would advise you "not to need" to clean it with steel wool in the first place, but is something burned up badly and you do not have heavy cleaning agents nearby steel wool is an acceptable solution.
If it is just for cleaning leftovers , usually soaking in soap water and a hard plastic brush does a very good job as well.
Remember that even thou Vitreous enamel is very hard and durable it may be damaged by overheating or crack if dropped.