Thomas, who at 40 years old is the oldest player in the NBA, is scheduled to have surgery on his right foot next week. He averaged 2.5 points and 2.3 rebounds in 39 games this season.
The Knicks will need to cut a player on their 15-man roster once their agreement with Singleton becomes official.
Singleton, a 6-8 forward, saw his last action in the league during the 2011-12 season when he played 12 games with the Washington Wizards, averaging 8.2 points a contest. Singleton played the 2012-2013 season in China with the Xinjiang Flying Tigers, averaging 15.1 points and 11 rebounds in 21 games.