Simple editor script that allows to set pivot for multiple slices of a sprite sheet automatically, so you won't have to do it manually. The sprite sheet has to be sliced beforehand.

SpritesheetPivotEditor.cs

using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(SpritesheetPivotEditor))]
public class SpritesheetPivotEditor : EditorWindow
{
    [MenuItem("Tools/Sprites/Pivot Editor")]
    static void OpenPivotEditor()
    {
        SpritesheetPivotEditor window = (SpritesheetPivotEditor)GetWindow(typeof(SpritesheetPivotEditor));
        window.minSize = new Vector2(300, 300);
        window.Show();
    }

    private void OnGUI()
    {
        EditorGUILayout.LabelField("Select the sprite sheet first");

        if (GUILayout.Button("Top Left"))
        {
            SetPivots(1);
        }
        if (GUILayout.Button("Top Center"))
        {
            SetPivots(2);
        }
        if (GUILayout.Button("Top Right"))
        {
            SetPivots(3);
        }
        if (GUILayout.Button("Left Center"))
        {
            SetPivots(4);
        }
        if (GUILayout.Button("Center"))
        {
            SetPivots(0);
        }
        if (GUILayout.Button("Right Center"))
        {
            SetPivots(5);
        }
        if (GUILayout.Button("Bottom Left"))
        {
            SetPivots(6);
        }
        if (GUILayout.Button("Bottom Center"))
        {
            SetPivots(7);
        }
        if (GUILayout.Button("Bottom Right"))
        {
            SetPivots(8);
        }
    }

    //  Pivot alignment:
    //
    //  +----------------+------------------+-----------------+
    //  |   TopLeft = 1  |   TopCenter = 2  |   TopRight = 3  |
    //  +----------------+------------------+-----------------+
    //  | LeftCenter = 4 |    Center = 0    | RightCenter = 5 |
    //  +----------------+------------------+-----------------+
    //  | BottomLeft = 6 | BottomCenter = 7 | BottomRight = 8 |
    //  +----------------+------------------+-----------------+

    //  +---------------+
    //  |   Custom = 9  |
    //  +---------------+

    static void SetPivots(int Alignment)
    {
        Object[] textures = Selection.GetFiltered(typeof(Texture2D), SelectionMode.DeepAssets);
        Selection.objects = new Object[0];
        foreach (Texture2D texture in textures)
        {
            string path = AssetDatabase.GetAssetPath(texture);
            TextureImporter ti = AssetImporter.GetAtPath(path) as TextureImporter;
            ti.isReadable = true;
            List<SpriteMetaData> newData = new List<SpriteMetaData>();
            for (int i = 0; i < ti.spritesheet.Length; i++)
            {
                SpriteMetaData d = ti.spritesheet[i];
                d.alignment = Alignment;
                newData.Add(d);
            }
            ti.spritesheet = newData.ToArray();
            ti.isReadable = false;
            AssetDatabase.ImportAsset(path, ImportAssetOptions.ForceUpdate);
            ti.isReadable = true;
        }
    }
}