Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13-inch baking pan.
Mix brown sugar, pecans, flour, and cinnamon together in a bowl. Pour in melted margarine; mix until evenly combined.
Combine cake mix, pumpkin puree, eggs, 1/2 cup plus 3 tablespoons water, canola oil, pudding mix, and pumpkin pie spice in a large bowl; beat with an electric mixer on low speed until moistened. Scrape sides of the bowl. Mix on medium speed until batter is smooth, about 3 minutes.
Spread half of the batter in the bottom of the prepared pan. Sprinkle half of the brown sugar mixture evenly on top. Pour in remaining batter; spread evenly. Sprinkle remaining brown sugar mixture on top.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 45 minutes.
