Hayes, 27, has signed a three-year deal at The Valley and will line up against the Iron as an Addicks player in a League One match in London on Saturday August 20.
"I'm really happy to get Paul on board, it’s a real coup for us. Paul did very well for Scunthorpe and has played in teams that have been promoted from League One," said Powell.
"He has always struck up good relationships with strikers like Gary Hooper and Martin Paterson, and he will come here and supplement the goal threat we already have in people like Bradley Wright-Phillips and Paul Benson."
Hayes will join up with his new teammates on June 1. "Can't wait to move home and get the season started," he wrote on Twitter.
Charlton confirmed talks with the former Norwich youngster earlier this week, and also indicated the Iron had made an attempt to bring him back to North Lincolnshire for what would be a third time.
It would be the second time this calendar year that Scunthorpe had tried to sign back the player who left on a Bosman free transfer last summer, having tried to bring him back on loan for the end of the season.