I mean you need to make an intake port from outside the grow into the area for the air cooled hoods to attach to. This gives you a cold air intake for your lighting rig. Keep your exhaust as it is and remove the carbon filter from the light rig.
Attach the ducting from where the filter attached to the new cold air intake port.
So you have your ducting going from your new intake port to your first air cooled hood, into the second and third and out of your exhaust using the 8"fan.
You said you had a 6" fan? Then connect this to the filter and mount it and create a second exhaust for the 6" fan to blow out of.
You then have the 8" only running your lights cooling.
The 6" with filter then becomes your exhaust.
o
J