Pierce chicken on both sides with fork; brush both sides with salad dressing. Using direct heat cooking method, grill chicken (HIGH) 5 min.; turn chicken breasts. Brush with additional dressing; continue grilling until juices run clear (3-5 min.). Cool; cut into thin strips. Cook pasta in boiling salted water just until tender (12-14 min.); drain. In large bowl, combine chicken, pasta, celery, green onion and grapes. In small bowl, combine oil and 1 1/4 tsp. thyme; toss into chicken mixture. In small bowl, combine mayonnaise, milk, honey, 1 tbsp. thyme, mustard and salt; fold dressing into salad. Refrigerate, covered.
To Serve: Toss walnuts into salad.