Dice the apples and place it in the bottom of the slow cooker.
In a mixing bowl, whisk together the pumpkin purée, evaporated milk, brown sugar, and pumpkin pie spice. Pour the pumpkin mixture over the diced apples.
In a separate bowl, stir together the cake mix, chopped walnuts, and rolled oats. Sprinkle the dry mixture over the ingredients in the slow cooker.
Melt the butter in the microwave and then drizzle it over the cake mix. Place the lid on the slow cooker and cook on high for 2.5 hours. After 2.5 hours, turn the heat off, remove the lid, and let the pudding rest for 15-30 minutes, then serve. Eat as is or top with whipped cream or vanilla ice cream.