If both systems are forced air systems it's possible to use the existing duct work; however, with out viewing you project, I would suggest having an air duct specialist evaluate your system to determine if modifications are necessary .
My parents did it with existing ductwork.
There should be a lever somewhere on the duct near the furnace. If it is open, the lever will be parallel to the duct. If the damper is closed, it will be perpendicular to the duct.
The heat it making the duct expand
Don't laugh, duct tape.
I heard that you should have ducts cleaned but I don't know how often. How often should furnace duct cleaning be done and how much does it cost?
Duct tape was invented to tape around joints in an air duct on a furnace. The ducts are square or round tubes that carry warmed air from the furnace to the floor registers in each room. If the joints are not taped, valuable warm air will leak out of the joints.
The duct work does not need to be different from the rest just because of the I beam. Just be sure there is room so you do not compress it.
I would like to do some furnace duct cleaning but I am nervous. Do I need a special vacuum to do this? Will I get soot all over my carpet if I mess up?
Try looking where the return air duct connects to the furnace, if not there the next place to look is in the return air grille in the living space. Either way it is somewhere in the return air duct between the grille and the furnace.
It all depends upon your setup. You need to find where the return duct is connected to your airhandler/furnace. You could have a return filter grill or there is a door/panel on your airhandler/furnace that must be opened to replace the filter.
the filter on a central unit is the same as the furnace filter usually located in duct work directly beside furnace
Buy the register, trace out the the hole for the part of it that will penetrate the duct, cut the hole and install the register then screw it in.