I thinks possible, but from the way Regina says “You should have stayed in Oz” I think the witch wasn’t in FTL or SB. Since Snow and Emma were in FTL its been like a few months, right? So the witch clearly hasn’t been there that long. As for how she could crossover from SB to FTL, we don’t know if the monkeys can world hop or not, if so they could have carried the witch. Or her BROOMSTICK could possibly be a world hopping device like Jeff’s hat :O
"I could have the giant duck as my steed!" --Daniel Radcliffe
Keeper Of Tamara's Taser , Jafar's Staff, Kitsis’s Glasses , Ariel’s Tail, Dopey's Hat , Peter Pan’s Shadow, Outfit, & Pied Cloak,Red Queen's Castle, White Rabbit's Power To World Hop, Zelena's BroomStick, & ALL MAGIC