That existing return of 18x8 will only provide 144 sq in of air no matter what else you cut into it. Whoever installed this poorly sized return duct in your home did not seem to have any clue as to duct sizing. Do not attempt to add any return in the immediate area of the furnace (it could upset the burner flame and or draw fumes back into the home, etc.)
The best and safest thing to do is to have a Hvac tech come into your home for a "hands-on" view and then give you a price on what is required to correct the situation. Make sure these quotes are from reputable heating firms, and better still get a 2 or3 qoutes from various firms, if possible.
Just curious as you claim this is a 175,000 btu furnace (input) with a hoizontal return of 18x8 into a vertical drop of 20x10. At the point of entry into the furnace 's fan compartment what size is it? In other words what size is the air filter at this entry point? .... 20x25 or larger?
Also, what size are these other return openings thru-out the home?