How do I substitute canned pumpkin for canned pumpkin pie filling?
The most common, almost canonical brand of canned "Pumpkin Pie Filling" in the US is Libby brand. While the canned pumpkin puree is just canned pumpkin, the Libby "Canned Pumpkin Pie Filling" also has sugar syrup, natural flavoring, salt and spices. So I would add the spices, salt and condensed milk in @Phrancis's recommendation to 30 ounces of pumpkin puree (probably without the cornstarch, there is probably cornstarch or other thickener in the rest of the recipe, if not, consider adding it as a part of the next step I am recommending here), more or less of the spices and sweetener to taste. The eggs and dairy (other than the sweetened condensed milk, which would serve in the substitution as the sweetener) are most likely in your recipe, not expected to be in the can of "Pumpkin Pie Filling"). 

You want a texture pretty similar to the unadulterated canned pumpkin product. If your substitution is a bit thinner than the the canned pumpkin was (as it should be, with the addition of sweetened condensed milk) simmer it a while to reduce. That can get your volume down to 30 ounces and intensify the pumpkin flavor, making your substitution probably better than the stuff for which you are substituting. Libby sells the pumpkin puree in 15, 29 and 106 ounce cans.

BTW, America's Test Kitchen does that reducing trick to canned pumpkin just routinely to give it a more intense pumpkin flavor and to eliminate "the taste of the can".