Changing size of the generic list classes

Hi, I have a generic array class and it works fine like below.

 public class Main : MonoBehaviour
 {
     [System.Serializable]
     public class arrayData
     {
         [System.Serializable]
         public class arrayData2
         {
         }
         public List<arrayData2> Levels;
     }
     public List<arrayData> Zones;
 }

However I can not add the same empty elements of it like new Zones element or Levels element inside a Zone. I tried many variations like Zones.Add(new Zones). I tried creating another array class and add it inside but then the class names cannot be the duplicate. I tried adding forexample the first element of Zones class as a new element but then later it is copying everything to the new class what the first element had. I can Remove it with xxx.RemoveAt() though. Also I want to see it on Editor so I do not want to take of the Serializable attribute out. Someone has any idea how I can add new Zones class? Thanks