Ardley has brought in the 27-year-old as cover with a host of Dons players currently sidelined through injury.
"Kelly can play anywhere across the back four," Ardley told the club's official website. "He came in and trained with us last week and looked very good in training. We feel that he will be a big plus for us during the final stages of the season.
"Kelly had a bad injury a few years ago when he was at Charlton and he was at Ipswich earlier this season, mainly as cover, but he was released in January.
"We have signed a player who was playing in the Championship at his peak so hopefully he can bring that experience for us.
"Kelly came highly recommended by several people who had worked with him and it is good that he is not another loan player.
"He is what we need at the moment because with Peter Sweeney, Jim Fenlon and Will Antwi all out and Curtis Osano still struggling we are a bit short."
Youga is available for Tuesday night's trip to Southend.