AnimationConverter.cs

using UnityEditor;
using UnityEngine;

public static class AnimationConverter
{
    [MenuItem("Assets/Animation/CopyAnimationClipAsLegacy", true)]
    public static bool ValidateCopyAnimationClipAsLegacy()
    {
        return Selection.activeObject is AnimationClip;
    }

    [MenuItem("Assets/Animation/CopyAnimationClipAsLegacy", false)]
    public static void CopyAnimationClipAsLegacy()
    {
        var clip = Selection.activeObject as AnimationClip;
        var copy = Object.Instantiate(clip);
        copy.legacy = true;
        AssetDatabase.CreateAsset(copy, "Assets/" + clip.name + ".anim");
        AssetDatabase.Refresh();
    }
}