Depending on your system, you may not need worry about providing 3,000 cfm to get your 600 cfm of ventilating air, if that's what you're concerned about. As long as you don't recirculate any of the room air, even if the supply contains only 20% OA, 600 cfm total air in is still providing ventilation if it is single pass and the room is equipped with a 600 cfm exhaust fan.