Outside air for Englander 28-3500?

I already have had an issue with airflow in my home due to the fact that attic fans that had been installed in the roof in order to prevent mold were creating a negative pressure situation because there were not enough soffit vents. I added more soffit vents and I also leave the access hole to my attic open (it's in the garage ceiling) along with the garage window in order to ensure that there is adequate airflow through the roof without causing negative pressure in the basement which was causing my oil furnace's chimney to back up in the house. However, once I install the add-on furnace I'm concerned that between the attic fans, clothes dryer, and bathroom fan that I'll once again have a problem. There is no "outside air kit" for the 28-2500, so how can I ensure that the stove is receiving enough air?

The chimney will be run out of a basement window opening which would be wide enough to accommodate two smaller openings, so what I envisioned initially was to have the thimble set to one side and instead of bricking up the opening with masonry as I had originally intended, I would run an 8" galvanized round duct from outside the house to the stove (not sure where to connect it)-then I'd install a grate or screen on the outside to prevent critters from getting in. Does this sound like it would work?