The move comes one day after the Predators assigned McGrattan to Milwaukee of the American Hockey League after he cleared waivers.
McGrattan will report to Calgary's AHL affiliate in Abbotsford, B.C.
This will be McGrattan's second stint with the Flames after playing for Calgary during the 2009-10 season, where he had one goal and three assists along with 86 penalty minutes in 34 games.
The six-foot-four, 235-pound Hamilton native has three goals, 13 assists and 456 penalty minutes in 214 games with Ottawa, Phoenix, Calgary and Nashville.
Piskula has played 10 NHL games with Los Angeles and Calgary over his seven-year professional career.