Brettm,
The practical matter is that you can't "Add an Antenna" because the modem is buried inside the laptop.
You can use a number of USB WiFi modems that will do what you want. They do work. I happen to be a fan of the Alfa 036 units. But they are nothing all that special they just have better RF for both receiving and transmitting than other I tried.
The on gotcha to the above is that you have to be sure that it lists a driver for your current operating system. It could otherwise be quite useless.
Matt