Stephen Carr has signed a deal with Birmingham until the end of the season.
The 32-year-old Irishman initially joined City on a short-term deal but he has impressed manager Alex McLeish.
The former Newcastle and Tottenham full-back has now signed until June, when they will again look at his situation.
"Stephen has brought a calmness and good positional, covering play to the back four," said McLeish.
"I never doubted his quality but a fear about the lack of games he had played this season before he arrived at the club has been dispelled by his performances."
Meanwhile, young striker James McPike will stay on at Solihull Moors for a third month on loan.