I did it. When I bought my 2006 Focus S it didn't have cruise. That was the only option I wanted that it didn't have. Ford makes a kit with the factory speed control computer and servo motor, clockspring, and steering wheel with built in buttons. Part of the deal when I bought the car was that I would buy the kit and then the dealer would install it. That way it's covered by warranty.
It works fine. The only difference is that the cruise light in the cluster doesn't come one when you activate the system. I think that I could fix that by running a wire from the servo to the cluster, but it doesn't bother me that much so I haven't tried yet.
Here's the link to where I found the kit. Search for "speed control" in the electronics section of the catalog. It says $465 for the kit but I don't remember paying that much.
I work with a guy who has installed a few Audiovox kits on cars he has. He really likes how they work but I wanted OEM steering wheel controls. Those are a lot cheaper. $100 to $200.
The guys at Focus Salvage could probably get you the computer and servo, clockspring and steering wheel, but then you'd be on your own for overlaying the missing wires into the harnesses. You could probably do it if you had access to some good schematics.