Growing up near Tuscaloosa, Alabama, Hallman aspired to play for Alabama under head coach Bear Bryant. Alabama did not recruit him, although Gene Stallings at Texas A&M did. He played on the Aggies' 1967 squad, which garnered a Southwest Conference championship and a Cotton Bowl Classic win. In the bowl game, Hallman intercepted two passes by Alabama quarterback Ken Stabler. Hallman graduated from the school in 1970.