The 29-year-old former Nottingham Forest defensive midfielder,who was a free agent, has signed an initial two month contract with Blues.
Manager Gary Rowett told the club's official website: “We knew Guy was out of contract and we invited him over to train for a week and he did quite well."
“We then got him over for another week just to make sure. It was a tough week and he got through all of that.
“He’s a big strong defensive midfielder and we feel that’s something we haven’t got so we’ve agreed a short term deal to bring him in and that’s reflected in what we’ve offered financially.
"It’s an opportunity for him to come and look at us and us to have a look at him and see if he can earn something.”
Moussi spent six seasons at the City Ground but was released by Forest in the summer.