I tried it once...it seemed like my duck was programmed to sit for a longer time so she didnt take the babies out of the nest!  Two of my Polish crosses adopted the chicks though...the duck hen continued to sit on nothing and at 32 days went off the nest...it was weird.
Can you find the nest and then set her on her own eggs?  If she was locked up she might brood them.  T
Thanks for the compliments on my new ones...that hen is just the best mama--even if she always has surprise babies!