If you are just talking about a momentary contact switch to replace (or work in addition to) the key starter, that would be no big wiring trick. But the "start" button that is on modern cars does more than that. For example, it knows if you are stepping on the brake pedal. It also tolerates a "momentary" press, in which you could let off the button immediately, before the car is started, and will keep the "start" function going until the car actually starts. Granted, most newer cars with this technology start immediately. I don't know about your truck, but tapping the start switch on mine momentarily would probably not produce an engine start most of the time.
I guess what I am getting at is, what level of integration are you looking for? It's going to be really hard to integrate all the functions of a modern start button, and it will require a computer. It's going to be really simple to run a secondary "start button" circuit to replicate the key-in-start-position. But if that's all you're going to do, my question would be, why bother?
81 F150 Flareside, Edelbrock Pro Flow4 FI, hydraulic roller 351W, E4OD, 4x4, BW1356
92 F150 RCLB 351W E40D BW1356 mostly stock