One of the reasons you don't have fat in powdered milk is that fat goes rancid much more quickly than the other ingredients. By drying it and getting out the fat it'll last longer.
There are several places you can get fat in a TEOTWAWKI, assuming you don't have livestock (chickens have schmaltz, for instance, or cow/sheep/goat's milk, etc.). You can gather nuts and seeds, eggs, game will have some fat (turkey, deer). Insects give both protein and fat. It can be done.