Cook first 4 ingredients in saucepan on medium heat 5 min. or until marshmallows are melted and mixture is well blended, stirring frequently. Cool completely.
Spread 1/2 cup nuts onto bottom of crust; drizzle with 1/4 cup caramel topping. Cover with half the pumpkin mixture.
Add 2 cups COOL WHIP to remaining pumpkin mixture; stir gently until blended.
Spread over pumpkin layer in crust.
Refrigerate 2 hours or until firm. Top with remaining COOL WHIP and caramel topping before serving.