Make collar for dish by cutting piece of waxed paper long enough to go around shallow (2-1/4-inch deep) 1-quart soufflé dish, overlapping ends of paper slightly. Fold paper lengthwise in half. Wrap around outside of dish, pulling tight so filling cannot run down between paper and dish. Secure ends of paper together with tape.
Gently stir together sorbet and whipped topping in large bowl until blended.
Stir together egg white powder and warm water in medium-size bowl for 2 minutes or until completely dissolved. Beat with mixer until soft peaks form.
Gently fold beaten whites into sorbet mixture. Spoon into prepared dish. Freeze until firm, about 3 hours. Let stand at room temperature 10 minutes before serving. Garnish with orange zest, if desired.
Egg white powder (pasteurized dried egg whites) is found in the baking section of supermarkets.