Im the past I know I have seen some keypad accessories for alarm systems that simply opened or closed a circuit after the correct code is entered.
Here is a link to a circuit that would do the trick.
A bit more searching , and I found this - http://www.hobbytron.net/9951.html they market it as a garage door opener, but it would work just fine for your purpose. You can also buy these things ready-to-go as garage door openers.
The only iffy part would be the timing. If you just want to enable / disable the starter circuit, still using the key to start the car, just throw in some relays, and your all set.
If you want to use this device instead of the key, you are going to have to monitor the voltage pattern of the cars electrical system, or tap the out of your tachomiter. (your rpms will jump, then settel as you crank and start the engine. ) both methods are comonly used by remote starter circuits.
My alarm / starter unit has an aux input. when I momentarially contact this aux input, the alarms auto start circuit kicks in and starts the car.
One final note - Tachomiter sensing is a bit more accurate. I recomend this to prevent over cranking.