How do I add preview devices to Interface Builder in Xcode to retrieve the device's code?
There's no way nor need to add devices there. The devices presented there cover all the available device resolutions you should care about and that's what is it for.

If you want to add simulated devices for you to test on you can go to 
Xcode->Window->Devices and click on the + on the lower left corner. 

I'm guessing that's the + sign you saw while searching about it