Thomas enjoyed a successful loan spell with the Blues towards the end of the 2012-13 season, scoring three goals in 11 appearances of which only five were starts.
The 27-year-old started his career in non-league football before joining Cheltenham in 2010, where he enjoyed his most prolific spell, scoring 18 goals in 41 league appearances.
Since then, Thomas has spent time at Crawley and Bournemouth, from where he went out on loan to Portsmouth and Blackpool before his stint with Birmingham.
Most recently he was with Rotherham, where he was part of the side that secured promotion to the Championship via the League One playoffs.
He scored six goals in appearances for the Millers who will now be among his opponents next season.