I'm importing a 147 Selespeed from Denmark. In the UK the winter pack comes as standard with the Selespeed, but in Denmark it does not. So in order to keep the car to full UK spec I needed to add the winter pack. The problem was that the car had already been built so I was stuck without the winter pack.
So I phoned round all my local Alfa dealers to see if they could fit the heated seats etc. after the car was manufactured with no luck at all. They said that they had never done this before and wouldn't even attempt it.
Anyway, I contacted the dealer in Denmark and they told me they could fit the heated seats with no problem at all, and gave me it at half price (as they messed up in the first place)
So what I'm trying to say is that I'm sure its possible but don't think many dealers will want to do it. Also the cost of the extras are very cheap when you specify them with the car, but if you want to add them after manufacter they will be quite expensive as the winter pack for me was nearly 4 times the price compared to the book price.
Hope this Helps!