Well, I guess the first question that comes to mind is "why" ? The second question is, are you talking about one on the dash to start the car each time, or one under the hood to bump the starter when tuning, etc.?
I kind of think you are talking about the one on the dash to start the car, and I would also guess it would be to give it that old Ford flavor, sort of like a step back in time? If that is the case, all you have to do is find the wire that comes out of your steering column that is hot only in the start position. This will be your starter signal wire. Just interupt it by putting a heavy duty momentary starter switch (push button type) in the line you just cut, so that when you push the button it sends 12 volts to the starter solenoid.
It does have a secondary benefit of providing somewhat of an anti-theft device to the car, especially if the button is hidden a little. A thief would turn the ignition, and possibly give up when the car wouldn't crank in the normal way. My Kid has this setup on his Honda, because the regular ignition switch went bad on the "start" portion, so he simply added a hidden switch that he pushes after turning the ignition switch to run. Every time I borrow his car I forget that little feature until it won't start, then I wake up.