InsertAnimation.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.Animations;

public class InsertAnimation : MonoBehaviour 
{
	private PlayableGraph graph;
	private AnimationPlayableOutput output;
	private Playable playable;

	[SerializeField] AnimationCurve curve = null;

	void Awake()
	{
		graph = PlayableGraph.Create ();
		output = AnimationPlayableOutput.Create (graph, name, GetComponent<Animator> ());
		output.SetWeight (0);
	}

	void OnDestroy()
	{
		graph.Destroy ();
	}

	public IEnumerator Play(float time, AnimationClip animClip)
	{
		if (playable.IsValid () )
			playable.Destroy ();

		// アニメーションのセットアップ
		playable = AnimationClipPlayable.Create (graph, animClip);
		output.SetSourcePlayable (playable);

		graph.Play ();

		// AnimatorからPlayableへ遷移
		for(float endTime = Time.timeSinceLevelLoad + time; 
			endTime > Time.timeSinceLevelLoad; ){
			float diff = 1 - (endTime - Time.timeSinceLevelLoad) / time;
			output.SetWeight ( curve.Evaluate (diff));
			yield return null;
		}
		output.SetWeight (1);

		// PlayableからAnimatorへ
		// AnimationClipPlayable再生中はAnimationControllerは止めておく
		GetComponent<Animator>().playableGraph.Stop();
		yield return new WaitForSeconds (animClip.length - time * 2);
		GetComponent<Animator>().playableGraph.Play();

		// PlayableからAnimatorへ
		for(float endTime = Time.timeSinceLevelLoad + time; 
			endTime > Time.timeSinceLevelLoad; ){
			float diff = (endTime - Time.timeSinceLevelLoad) / time;
			output.SetWeight ( curve.Evaluate (diff));
			yield return null;
		}
		output.SetWeight (0);

		// アニメーションのクリーンナップ
		playable.Destroy ();
		graph.Stop ();
	}
}

Sample.cs

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

public class Sample : MonoBehaviour 
{
	[SerializeField] InsertAnimation play = null;

	private Coroutine coroutine;
	
	public void Play(AnimationClip clip)
	{
		if( coroutine != null)
			StopCoroutine (coroutine);
		coroutine = StartCoroutine (play.Play (0.2f, clip));
	}
}