Ford tends to be cheap when it comes to including extra hardware, so I wouldn't expect the antennas to be there if it wasn't an option. You could try, but I won't gaurentee it will work.
1) Gather all the remotes you plan on using with your car.
2) Get in car, fasten seatbelt, and close doors to make sure it doesn't beep when you turn the ignition to "Run"
3) Place key in the ignition.
4) Turn the ignition to "Run" (Not Start), then back to "Off" at least 4 times in 6 Seconds, stopping at "Off"
5) The car will make one "BONG!" tone, like when you leave the car door open with the keys in the ignition.
6) Hit "Unlock" on each remote you wish to use. Wait about 1 second between each remote. You only have a window of 10 seconds to add your remotes. Only 4 can be added at a time, and if you add any remotes, then all the ones that used to work for the car that you did not program will not work.
Sit in the car with the doors shut and unlocked, and with your seat belt fastened to avoid interference from the seat belt reminder chime.
Put the key in the ignition, and turn it from position one ("Off") to position three ("On") rapidly eight times within 10 seconds, ending in position three.
The Ford Focus doors will lock and unlock automatically, signaling that the car is in remote keyless entry programming mode.
Within 20 seconds, press any button on each keyless remote. After each remote is programmed, the car doors will automatically lock and unlock to confirm successful programming. When all remotes have been programmed, turn the ignition to position one. The doors will automatically lock and unlock, confirming that remote keyless entry programming is complete