Air conditioners cannot be added to most furnaces, but they can be added to the ductwork to which the furnace connects. Those looking to save space can purchase integrated furnace and air conditioners.
Air conditioners work on an entirely different principle than furnaces, so there's little in a furnace that can aid in air conditioner operation. However, many of the principles that guide duct installation for furnaces are similar for air conditioners, and it's fairly inexpensive to add an air conditioner to homes with existing furnaces. In homes with boilers, on the other hand, central air conditioner installation is far more extensive and expensive since ducts need to be run through the home.