Scharner, 32, joins Wigan on loan to return to the club where he first made his name in English football.
The midfielder first signed for Wigan from Brann in January 2005, going on to make 145 Premier League appearances before a move to West Brom in 2010.
He signed for Hamburg last summer but has been restricted to only one Bundesliga start at the German club.
“It really is fantastic to be back," Scharner told the club's official website. "Wigan will always have a big place in my heart, so to be asked to come back and help the team now is something really special for me.”
Scharner is Wigan's fourth signing of the January window following Roger Espinoza, Angelo Henriquez and Joel Robles.
Roberto Martinez's side are also closing in on Barnsley defender Stones, 18, who has made 26 appearances for the Championship club this season.
A short statement from Barnsley said: "Barnsley Football Club have today given permission for Wigan Athletic to speak to John Stones."