Bonjour James,
Alan (Jim) is right here : 'dans une sorcière' would imply something like 'inside a witch', which sounds very strange to French ears.
The expression is : se déguiser en [qch] = to dress up as [sth]
Il se déguise en père Noël = he dresses up as Father Christmas
Elle s'est déguisée en fantôme = she dressed up as a ghost
I hope this is helpful.
Bonne journée !