it would be easy to introduce her. we know we have only gotten familiar with a subset of the survivors. look in the season finale, that guy who was asploded by the dynamite had never been seen before, but he had a lot of lines, and could have easily been developed into a character if desired. she just needs to be another survivor who was unimportant until they need her. i know at some point they mention how many survivors there were, i want to say 40 or 60?
or of course there could be another set of survivors... though we dont know the size of the island, i doubt this scenario because to survive this other group would have had to build a fire which would have been seen and investigated.