In a large mixing bowl, combine the dry cake mix, pumpkin puree, and 1/2 cup of oil. I use canola but use whatever oil you prefer for baking. I just used a silicone spatula instead of breaking out the mixer, but the mixer will be needed later on for the filling.
One trick I learned from watching Martha Stewart for several years is, whether you're using a pastry bag or a storage bag, place it in a tall drinking glass with the top folded over the sides so that the bag stays open and upright while you're filling it. Once you get the bag pretty full, take it out of the glass and snip off one of the corners and work the filling down to the opening.