unity how to automate changing sprite editor setting

this can be done with an Editor script and the AssetImporter:

1. Start by creating a TextureImporter

// spritesheetPath with Assets folder as root TextureImporter importer = AssetImporter.GetAtPath( spritesheetPath ) as TextureImporter;

2. Get the SpriteMetaData array using importer.spritesheet
3. Edit the pivot properties (pivots are relative to sprite's rect, in [0-1] range as shown in the inspector), set the alignment to Custom and replace/re-assign the spritesheet array.
4. Flag the asset using EditorUtility.SetDirty ( importer ).
5. Commit your changes with importer.SaveAndReimport ()

Here's a complete script with all the bells and whistles:

using UnityEditor;
using UnityEngine;

public class SpritesToolsWindow : EditorWindow {

    [MenuItem ( "Window/My Utilities/Sprites Tools" )]
    public static void SpriteTools () {
        SpritesToolsWindow spriteTools = GetWindow<SpritesToolsWindow> ( "Sprites Tools", true );
        spriteTools.Show ();
        if ( Selection.activeObject != null && Selection.activeObject.GetType () == typeof ( Texture2D ) )
            spriteTools.spritesheet = ( Texture2D ) Selection.activeObject;
    }

    public Texture2D spritesheet;

    private Vector2 m_CommonPivot;
    private string m_Log;
    private void OnGUI () {

        spritesheet = EditorGUILayout.ObjectField ( "Spritesheet", spritesheet, typeof ( Texture2D ), allowSceneObjects: false ) as Texture2D;
        m_CommonPivot = EditorGUILayout.Vector2Field ( "Pivot", m_CommonPivot );
        using ( new EditorGUI.DisabledGroupScope ( spritesheet == null ) )
            if ( GUILayout.Button ( "Edit sprites" ) )
                EditSprites ();

        m_Log = EditorGUILayout.TextArea ( m_Log );
    }

    private void EditSprites () {
        if ( spritesheet != null ) {
            string spritesheetPath = AssetDatabase.GetAssetPath( spritesheet );

            if ( !string.IsNullOrEmpty ( spritesheetPath ) ) {

                TextureImporter importer = AssetImporter.GetAtPath( spritesheetPath ) as TextureImporter;

                if ( importer != null && importer.spritesheet != null && importer.spriteImportMode == SpriteImportMode.Multiple ) {
                    SpriteMetaData[] spritesMetaData = importer.spritesheet;
                    for ( int i = 0; i < spritesMetaData.Length; i++ ) {
                        SpriteMetaData metaData = spritesMetaData [ i ];
                        metaData.pivot = m_CommonPivot;
                        metaData.alignment = ( int ) SpriteAlignment.Custom;
                        spritesMetaData [ i ] = metaData;
                    }

                    importer.spritesheet = spritesMetaData; // seems like this setter internally change stuff (needed)
                    EditorUtility.SetDirty ( importer );
                    importer.SaveAndReimport ();

                    m_Log += string.Format ( "Edited {0} sprites in {1}\n", spritesMetaData.Length, spritesheetPath );
                    return;
                }

                m_Log += "Texture is not a spritesheet.\n";
            }
        }

        m_Log += "Could not complete action.\n";
    }
}

Cheers!