Heat oven to 375°F. Unroll crust on ungreased large cookie sheet. In medium bowl, mix pumpkin, brown sugar, 1/2 teaspoon maple flavor, the pumpkin pie spice and salt until smooth.
Spread pumpkin mixture over half of pie crust to within 3/4 inch of edge. Brush edge with water; fold crust over filling. Carefully move turnover to center of cookie sheet. With fork, press edge to seal; prick top several times.
Bake 23 to 30 minutes or until golden brown. Cool 5 minutes. Remove from cookie sheet to cooling rack.
In small bowl, mix powdered sugar, 1/4 teaspoon maple flavor and the milk until smooth and desired drizzling consistency. Drizzle over warm turnover. Sprinkle with chopped walnuts. Let stand 10 minutes. Serve warm or cool.