SpriteAssetPostprocessor.cs

using UnityEditor;

namespace Kogane
{
    internal sealed class SpriteAssetPostprocessor : AssetPostprocessor
    {
        private void OnPreprocessTexture()
        {
            var textureImporter = ( TextureImporter )assetImporter;

            if ( textureImporter.textureType != TextureImporterType.Sprite ) return;
            if ( !textureImporter.importSettingsMissing ) return;

            textureImporter.spriteImportMode = SpriteImportMode.Single;
        }
    }
}