outside air

Avalon "perfect fit" installed professionally in a brick fireplace. I am disappointed there is no outside air option. The front of the "perfect fit" sits right on top of the vent opening for the outside air previously installed for the fireplace. I propose to punch one inch holes in the bottom of the perfect fit and close the grille that is the current access to the room air for combustion and for reheating. Then outside air would substitute for the room air for both combustion and for reheating. Alternatively, I could adapt the air handling so that the outside air only goes to combustion and room air would be reheated. The thermodynamic trade offs are beyond me. Which is "more efficient" xfer of energy to heat in the room? I have heard an argument to use outside air for reheating because it would add "positive pressure" to the air in the house (thus reducing drafts coming into the house). But most inserts use room air for reheating. What to do?