Will a female duck foster duckling if she isn't a mother.
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She'll most likely attack the duckling. In general, ducks will not accept ducklings they did not hatch out themselves. That said, you can slowly introduce them to each other in a safe way so that they will become friends over time.
How old is the duckling? If it still needs heat (less than 4 weeks old, depending on your location and weather conditions) you may have to take care of it yourself until it is old enough to be outside next to your other duck. In that case you'll need to get a heat lamp and set up a brooder, and hopefully try to find it a friend or two of the same age. Ducks do not do well alone.
Once it's old enough to be out next to your older duck, you can make small pen for the duckling(s) next to your female. This way they are protected from being attacked by her, but they can all see/hear each other and get used to each other. Over time they will become used to each other and can be in together.