How Do I Convert String to String Array In List.Add?

Hello! I'm working with a password system for "Computers" in my game. I want to make it so you can add and save passwords to the system. After a lot of tedious work, I got to a point where I use string.Join to turn my list of strings into one string. I then load my string, where I want to split it and turn it into a list. I will then use that list and set the list of codes. The only issue is I can't split the string in the List.Add function, and I don't believe I can add an array to a list. If anyone could help, here is the code: private const string seperator = "!SEPARATE_VALUES!";

 public GameObject disableSelf;
 public GameObject loadNextOne;
 public GameObject loadNextTen;

 // Stored Strings
 public List<string> codesLvlOne;
 public List<string> codesLvlTwo;
 public List<string> codesLvlThree;
 public List<string> codesLvlFour;
 public List<string> codesLvlFive;
 public List<string> codesLvlSix;
 public List<string> codesLvlSeven;
 public List<string> codesLvlEight;
 public List<string> codesLvlNine;
 public List<string> codesLvlTen;

 //Load Strings
 private List<string> codesLOne;
 private int i = 0;

 void Start () {
     string CodesOne = File.ReadAllText(Application.dataPath + "/SaveData/passwordListOne.txt");

     string[] splitCodesOne = CodesOne.Split(new[] { seperator }, System.StringSplitOptions.None)

     while (i <= splitCodesOne.Length) {
         codesLvlOne.Add(splitCodesOne[i]);
         i++;
     }
 }

 void OnApplicationQuit () {
     SavePasswords();
 }

 public static void SavePasswords () {
     List<string> CodesLOne = string.Join(seperator, codesLvlOne);

     File.WriteAllText(Application.dataPath + "/SaveData/passwordListOne.txt", CodesLOne);
 }