Re: Retro fitting winter pack..
Probably possible, but not too practical. The heated seats would need their own wiring(high power) and control systems. The install in the factory would probably also include installing the correct electrical systems. EG I had an old sierra with heated leather seats and it had a higher output alternator than a non-heated seat car as well to allow the car to run in winter without emptying the battery. The seats would also cost a fair bit, add to that the cost of install and electrics, probably cheaper in the long run to chop your car in for one in the spec you want.