can i add cold air returns to my second floor without placing ductwork inside the walls? ie using the wall studs on interior wall as ductwork and then connecting to the cold air return ductwork in my basement?

It has been done for many decades. It misses the idea of sheetmetal to act as a fire channel liner; but it is done.

www.heatpro.us energy businessmen's knowledge

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.

I have three bedrooms upstairs do i need one for each room or will one in the hallway be enough? also I have an old wooden grate return aprox. 2ft by 3ft on the floor of my living room. I'm re-carpeting and want to make this smaller is there a risk of my furnace not performing up to standard if I do so? There are two other cold air returns on my first floor.

Returns are mostly a matter of totaling up to about the same cross-section as the supply main.
so if you have an 8 x 24 inch duct cross section, three returns will total up to 196 square inches of cross-section, like (3)returns that are each 8 x 8.

If you have a return in the hall and close the doors, you have to have a way to get all the air you need to return past those doors. If you undercut the door and it is 30-inches wide, then you have 30 square inches for air to come out - half an 8x8 = 64.

It is simple applied physics, not guesses.

www.heatpro.us energy businessmen's knowledge

Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.