Rangers have confirmed the signing of Dundee United striker Jon Daly.
St Mirren defenders Sean Kelly and Jason Naismith have signed new one-year contracts with the club.
The Central African Republic international joined up with Mick McCarthy's squad on Monday and has been included in the squad for Tuesday night's Suffolk Premier Cup tie at Walsham Le Willows.
Youga, who started out at French club Lyon, spent six seasons at Charlton from 2005 and went on to make 71 appearances for the Addicks.
The 27-year-old, who has had loan spells at Bristol City, Bradford and Scunthorpe, was released from Charlton in the summer of 2011 following a series of knee injuries.
He joined Yeovil in January on a short-term deal, but left the Glovers by mutual consent two months later and made his international debut for the Central African Republic in a 2-0 win over Botswana last summer.