Thomas, 25, spent time with the Robins during the curtailed 2020/21 season, having previously made over 50 appearances for Mansfield Town in League Two and also featured for Basford United in the NPL Premier.
The Robins have also brought in striker Dan Bradley.
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Bradley, 30, has extensive experience at step two and three having featured for the likes of Alfreton Town, AFC Fylde, Kidderminster Harriers, Nuneaton Borough and most recently Tamworth.
Both have signed deals until the end of next season.