If you're actually storing preferences, just use NSUserDefaults. It works the same (from an API perspective) inside a sandbox as outside. It's really best to pretend that preferences aren't stored in plists though; relying on that implementation detail can have all sorts of unfortunate side effects on recent OSs, so just use the API.
If you're trying to modify arbitrary plists rather than storing preferences, don't use NSUserDefaults. Use NSPropertyListSerialization and read/write them directly.