Hi WillowCat
I tend to make all my cakes from scratch, but you can use pumpkin and cake mix together so you should be fine. Looking at the recipes, most people are opting for spice cakes, and ginger cake mixes to add the pumpkin to which I think is because the flavours complement each other.
There are a few other recipes, but they use a yellow cake mix and a whole host of additional ingredients like the cinnamon and pumpkin spices or vanilla. Some of the recipes I've spotted are calling for a 15 oz can like yours to be added to the cake mix, and then a few other ingredients or spices have been added for good measure. Each recipe has varying amounts so you may wish to experiment with the pack you have and make up your own recipe...