Unity上の画像アセットに対して自動生成されたMipMapを個別の画像ファイルとして抽出するエディタ拡張です。各画像に手を加えた後 http://baba-s.hatenablog.com/entry/2019/06/04/084000 で再読込し、1枚のテクスチャとして使用できます

MipmapExtractor.cs

using System;
using System.IO;
using UnityEngine;
using UnityEditor;

public class MipmapExtractor : EditorWindow
{

    [Serializable]
    public class MipmapExtractorParams : ScriptableObject
    {

        [SerializeField]
        private Texture2D _sourceTexture;

        public Texture2D SourceTexture
        {
            get { return _sourceTexture; }
            set { _sourceTexture = value; }
        }

    }

    private MipmapExtractorParams _params;

    [MenuItem("Assets/Mipmap Extractor")]
    private static void Create()
    {
        var window = GetWindow<MipmapExtractor>("MipmapExtractor");
        window.minSize = new Vector2(320, 320);
    }

    private void OnGUI()
    {
        if (_params == null)
        {
            _params = MipmapExtractorParams.CreateInstance<MipmapExtractorParams>();
        }

        using (new GUILayout.HorizontalScope())
        {
            using (new GUILayout.VerticalScope())
            {
                EditorGUILayout.LabelField("Source Texture");
                _params.SourceTexture = (Texture2D)EditorGUILayout.ObjectField(_params.SourceTexture, typeof(Texture2D), false, GUILayout.Width(70), GUILayout.Height(70));
            }
        }

        using (new GUILayout.HorizontalScope())
        {
            if (GUILayout.Button("Extract"))
            {
                if (!_params.SourceTexture.isReadable)
                {
                    EditorUtility.DisplayDialog("Error", "Check \"Read / Write Enabled\" of the texture and apply the change, then try again.", "OK");
                    return;
                }
                Extract();
            }
        }
    }

    private void Extract()
    {
        var src = _params.SourceTexture;
        var srcPath = AssetDatabase.GetAssetPath(src);
        var dir = Path.GetDirectoryName(srcPath);
        var prefix = Path.GetFileNameWithoutExtension(srcPath);
        var pathFormat = "{0}/{1}.mip{2}.png";

        var w = src.width;
        var h = src.height;
        for (int i=1; i<src.mipmapCount; i++) {
            w >>= 1;
            h >>= 1;
            var dst = new Texture2D(w, h);
            dst.SetPixels(src.GetPixels(i));
            var path = String.Format(pathFormat, dir, prefix, i);
            File.WriteAllBytes(path, dst.EncodeToPNG());
        }
        AssetDatabase.Refresh();
    }

}