"He is here and available today," manager Fredi Gonzalez said. "He will be in the bullpen."
Volstad was 4-4 with a 3.36 ERA in 15 starts for Carolina. The 21-year-old was the 16th overall selection taken in the 2005 draft.
Volstad takes the roster spot of left-hander Taylor Tankersley, who was optioned to Triple-A Albuquerque after Florida's 12-6 loss to Colorado on Saturday night.