Depending on the circumstances, a pregnant dog technically could nurse another dog's puppies -- but that doesn't mean that she should. The expectant mother needs that milk for her own pups, and she only has a limited window in which to produce it. If another dog's puppies aren't getting the nutrition they need from their own mother, they should find an alternative food source other than a pregnant dog.
Expectant mothers don't start producing milk until seven days before giving birth, at the earliest -- they may not start until the day before. This gives her a very limited period of time to produce the milk her puppies will need almost immediately after being born, and she shouldn't lose it to other pups. Mother dogs who have already given birth and are currently nursing their own litters, however, are generally receptive to the addition of other puppies. Introducing your puppies to a mother who is nursing, particularly one with a smaller litter, is a safer route for getting them the nutrition they need. If no such mother is available, contact your vet immediately about hand-feeding the puppies yourself with formula.