NSDictionary has a function for reading and writing plist files. As does NSArray. No conversion required.
Although to read data in preference files, you should be using NSDefaults and not directly interacting with the preference files. This has been an Apple guideline since Mac OS X 10.0, Mac OS X 10.9 was the first OS version that I’m aware of which caches the preferences and manually editing the files (even deleting them) can result in weird things happening to the application.