If you have a single split system air conditioner unit, then you should not need to add more evaporator coils into the system. The split system is divided between an outside piece, which contains the compressor and condenser, and the indoor part which holds the evaporator. This is in the main supply duct of the home, so adding more could simply damage the supply duct, rather than produce cooler air. In order to ensure that you don't damage the ducts, you should consider adding evaporator coils near the termination point.
Step 1 - Open the Wall
In order to get at the air ducts for your system, you will need to open up the walls of the rooms where you want to install the evaporator coils. A single split system air conditioner usually has the same ducts as your heating system, which could mean that you will be connecting your evaporator coils to the warm air supply. Look at a plan of your home to decide which are the best ducts to alter, and then knock through the wall using a hammer and chisel.
Step 2 - Add the Evaporator Coil
Take your evaporator coil and install it by removing a section of the pipe. You can disconnect the pipe at a valve or bend, and put the evaporator coil into position. You may need to cut down the size of the pipe before you can connect it back up. Once the coil is in place, screw it into position tightly by turning the connection with your wrench. Make sure that you have all of the necessary wires on the outside of the coil.
Step 3 - Connect the Coil
You will now need to take the coil, and connect it up to the main evaporator coil. You can do this by running enough wire through your rooms to carry it to the main coil. When this has been done, you will need to find the wire connections which turn the power on to your coil, and the connections for controlling the amount of cool air provided. When you have done this, splice the wires from the main power supply with the ones from your new evaporator coil, until you have both coils running off the same source.
Step 4 - Finishing Touches
Once you have added the wires to the coil, you should give the system a test run to make sure that it works. Leave it to run for an hour or two, and check the wires at both ends. When satisfied that the system is working safely, you can add some caulk or pipe tape around the edges of the coil, and then plaster up the hole you made in the wall. Repeat this process for all of the evaporator coils that you wish to add, although you should be careful of adding too many wires to the main coil unit.