[Serializable]
 public class PlayLegacyAnimation : AnimationStepBase
 {
     public override string DisplayName => "Play Legacy Animation";

     [SerializeField]
     private Animation animation;

     public override void AddTweenToSequence(Sequence animationSequence)
     {
         animationSequence.AppendInterval(Delay);
         animationSequence.AppendCallback(
             () =>
             {
                 animation.Play();
             }
         );
         animationSequence.AppendInterval(animation.clip.length);
     }
 }

Absolutely! The same as the step, you can add any new DOTween action by extending DOTweenActionBase. In order to avoid any performance issues all the tweens are created on the PrepareToPlay method on Awake, and are paused.

[Serializable]
public sealed class ChangeMaterialStrengthDOTweenAction : DOTweenActionBase
{
    public override string DisplayName => "Change Material Strength";
        
    public override Type TargetComponentType => typeof(Renderer);

    [SerializeField, Range(0,1)]
    private float materialStrength = 1;

     public override bool CreateTween(GameObject target, float duration, int loops, LoopType loopType)
     {
        Renderer renderer = target.GetComponent<Renderer>();
        if (renderer == null)
            return false;

        TweenerCore<float, float, FloatOptions> materialTween = renderer.sharedMaterial.DOFloat(materialStrength, "Strength", duration);
        
        SetTween(materialTween, loops, loopType);
        return true;
    }
}

You can use the Custom ease to define an AnimationCurve for the Tween.

• None Don't do anything on the AnimationSequencer Awake method
• PrepareToPlayOnAwake This will make sure the Tweens that are from are prepared to play at the intial value on Awake.
• PlayOnAwake Will play the tween on Awake.*

This package is available on OpenUPM: https://openupm.com/packages/com.brunomikoski.animationsequencer

To add it the package to your project:

• open Edit/Project Settings/Package Manager
• add a new Scoped Registry:

  Name: OpenUPM
  URL:  https://package.openupm.com/
  Scope(s): com.brunomikoski
            com.demigiant
  

• click Save
• open Package Manager
• click +
• select Add from Git URL
• paste com.brunomikoski.animationsequencer
• click Add

You can also add it directly from GitHub on Unity 2019.4+. Note that you won't be able to receive updates through Package Manager this way, you'll have to update manually.

• open Package Manager
• click +
• select Add from Git URL
• paste https://github.com/brunomikoski/Animation-Sequencer.git
• click Add