He imitated Isabella without access to her body (as far as we're aware).
He may have wanted the body so as to simply propagate the idea that he was Locke, but then why didn't he do a better job of hiding the real Locke's body? He just left it in the plane; the one place the survivors of a plane crash are more or less guaranteed to look!
I'm stil really confused by this aspect of the story. Someone untangle my mind, please :)