Heat oven to 350°F. Spray 15x10x1-inch pan with cooking spray, or grease and flour pan. Place cereal in resealable food-storage plastic bag; seal bag and crush with rolling pin or meat mallet (or crush in food processor).
In large bowl, beat oil, 1/2 cup orange juice, the pumpkin and eggs with electric mixer on low speed until well blended. Stir in crushed cereal; let stand 10 minutes.
Beat in remaining bar ingredients except walnuts on low speed until well blended. Stir in walnuts. Spread evenly in pan.
Bake 35 to 40 minutes or until top springs back when touched lightly in center. Cool completely, about 1 hour.
In small bowl, mix powdered sugar and enough of the 4 to 6 teaspoons orange juice for desired drizzling consistency. Drizzle over bars. For bars, cut into 8 rows by 4 rows.