I think its possible, the only thing is, how would Wicked have crossed back to EF after magic arrived in SB?
I feel it is a big plot hole that Regina activated this curse with several “worst of the worst” types and when magic and memories restored, not ONE of them, other than King George in his limited way, set out to cause havoc. Even he didn’t set out to get revenge on Regina, but I find it hard to believe no one would want that. So if Wicked were in SB, but managed to escape (perhaps her Yellow Brick Road?) I’d love the storyline for referencing the fact that Regina cursed a lot of good as well as BAD people, and not everyone was loving it.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass