gistfile1.cs

using UnityEditor;
using System.Collections.Generic;

public class Question9
{

    private static List<string> scenePaths = new List<string>();

    [MenuItem("File/AdditiveScenes")]
    static void Additive()
    {
        scenePaths.Clear();

        while (true)
        {
            string sceneParh = EditorUtility.OpenFilePanel("Select Additive Scene", "Assets/Question8", "unity");

            if (string.IsNullOrEmpty(sceneParh))
            {
                break;
            }
            scenePaths.Add(sceneParh);
        }

        foreach (var scenePath in scenePaths)
        {
            string projectRelativePath = FileUtil.GetProjectRelativePath(scenePath);
            EditorApplication.OpenSceneAdditive(projectRelativePath);
        }
    }

}