it's gonna get expensive quick. likely gonna need to swap out your SCCM and clockspring, and of course the steering wheel itself. then you gotta change a bunch of settings in various modules with FORScan to tell it it has a heated steering wheel now. and even after all that, there's no promise it will work perfectly. it's one of those things where buying it with the options you want is easier and cheaper than retrofitting them later
Just an FYI - My Mach 1 steering wheel came from Flat Rock equipped with the heating element w/wiring, even though I didn't order the option. Ford's no dummy, ForScan can't override the BCM to activate it.
Basically, yes, you can add one. You will need to replace the steering wheel, the clock spring, the SCCM, and do some basic wiring to power the new SCCM. Then it's just a few FORScan changes to enable it.