How can I thicken an already cooked pie filling? I made a coconut cream filling using 2% milk as that was all I had.  Now, more than 6 hours later, it still has not set although it was thick while I was waiting for it to cool before pouring into the prebaked crust.  Does it need to stay overnight to set?  Can I pour it back into a saucepan and warm it up and add more cornstarch?  I hate to waste the ingredients.
If your filling is similar to a custard, consider adding a lightly whisked egg and baking on medium-low heat for maybe 20 minutes. If you must stove-cook it, stir only enough to combine the ingredients well, as whisking too much will weaken/break the proteins. 

Protein is the key to thickening things like custards and other fillings. Coconut milk is lower in protein than cow milk. The heat will allow the proteins to coagulate and form a sort of net that traps the rest of the ingredients, causing a thickening action. Refrigerating it for a short time will also contribute to solidifying the protein network and help set it.