In a large bowl, mix together spice cake mix, pumpkin puree, eggs, and melted butter. Slowly add in milk until batter is pourable (it might not be necessary to use the entire half cup). Spread cake mixture in the bottom of a greased 9x13" dish.
Bake according to package directions (usually 25-30 minutes).
Once the cake is out of the oven, allow it to cool for 10-15 minutes. Take a fork and poke holes throughout the cake.
In a separate bowl, mix together sweetened condensed and evaporated milk. Pour this mixture over the top of the cake, filling the fork holes.
Frost the cake with the cream cheese frosting and allow to cool in the fridge until ready to serve.