It would be tremendously expensive to retrofit. You'd be better off just trading in for a vehicle that already has the option. This is one of those things that you get from the factory or not at all.
According to the manual, the recognition works off stored maps and the camera behind the mirror. Seems like it should work automatically. Search the manual for "Speed sign recognition" to get more info.
That photo is what the clearance looks like on my stock FX4 with the 255 tires. I can't get a finger in there, so it's less than ½". I suspect that a wider tire with the same offset might rub.
Those covers originally only came on the Lariat models. I don't know if they're still being installed, but if you live in an area that has a mouse problem, those covers make a very nice mouse house on top of the engine. Obviously, not desirable.
