add on starter to factory 04 expedition

Want to add a remote starter to my wife's factory keyless/alarm on her 04 Ford Expedition.  How do I send the input to the aftermarket starter module and how will she activate the remote starter thru the factory fob?  Is there a module I need to buy to do this?  She wants to keep the factory alarm because of the number keypad on the outside of door.

Thanks, David

draasch, please tell me more about the double or triple pulse.  She wants to keep the factory alarm because of the keypad on the outside of the door.  My girls use this keypad to unlock the doors when they need something out of it and she doesn't want to sacrifice that.  How can I program the factory remote?  Please let me know more information.

Thanks