My American Buff goose adopted 2 two-week-old goslings when her 5 were only days old. We had them where she could see them for awhile, and then let them go in with her. She gave them some odd looks (you could just tell she was wondering how two of her goslings got so big so quickly) but she took them in as her own almost immediately. She was more worried about us than the extra goslings.
I'd set up a safe area for her to look at them and sit back and watch her for awhile -- see what she does. But, as long as they are making baby gosling noises still, then I think she'd do fine! Just make sure you watch them at first . . .