Retrofitting it might be completely impossible.
Problem 1
MyLink, like many GM radios cross-pollinate with the OnStar equipped in the vehicle.
So far as we know, it at least uses its mic and possibly the BT receiver is in the OnStar unit.
Worst case scenario is it's completely tied to an OnStar unit that only works in the intended vehicle.
The RPO codes for ordering a Sonic mention that if you order the OnStar delete (a fleet option) it automatically makes the vehicle ineligible for MyLink. (Even in an LTZ where it would be standard)
Problem 2
The MyLink security mentions that one of its theft deterrent features is that it only functions when connected to the vehicle with the proper VIN (I could be wrong, but I think that's the statement, maybe the ECU holds VIN info?)
ps, GM, if that's an ACTUAL theft deterrent feature and WE don't know it, how does it stop the thief BEFORE they break into a car?
If the crackhead that stole my radio doesn't know til he gets to his "fence" that it's now worthless it doesn't help me at all.
Chevy Volt owners are asking the same thing, one poor bastard bought 2 2012's and *his came with MyLink but his wife's didn't,... I don't envy him at all.
(Note, it has a nav unit but she still can't get MyLink)