Bring a large pot of lightly salted water to a boil. Cook orzo in the boiling water, stirring occasionally until cooked through but firm to the bite, about 11 minutes. Drain and transfer to a bowl; refrigerate until chilled.
Whisk mayonnaise, Dijon mustard, sour cream, orange juice, curry powder, salt, and pepper together in a bowl until dressing is smooth; cover with plastic wrap and refrigerate until chilled, at least 30 minutes.
Mix pineapple, chicken, celery, apple, grapes, and green onions together in a bowl. Stir orzo and dressing into pineapple mixture. Gently fold oranges into salad and top with pumpkin seeds.