At 5 days old, I would be leery of trying that... ducks are much more finicky about adopting... the ducklings tend to imprint quickly so they may not recognize her as a momma... and if they don't stop crying she may attack them in frustration...
Ducks seem to communicate a lot more with their brood before they hatch, bonding while still in shell... if they were just hatched, I might try it... but I know my Calls have wanted to try to take in other ducklings, but it got rather frantic when the older ones kept freaking out...