How long did she wait for a clutch before she started to brood? In other words, how many eggs is she sitting on, aside from the duck eggs you added? The refrigerated eggs "can" hatch, but with drastically lower chances. The unrefrigerated 2 have a pretty high probability of hatching if they are fertile. As for continuing to sit on them after her own hatch, that's really up to her. But it's unlikely. Once hers hatch, she will be ready to care for her own and consider the unmatched duds or a loss after only a couple days. She may even kick them out of the nest all together. Being duck eggs take much longer than the 21 days for her, i don't see it. You will likely need to incubate them for the rest of the time they have left.