Actually they can be fitted after: but the work should be done properly, because you have to dig some holes in the roof, if they're not sealed in a good way, rust can be a problem too.
Better if you have something you can attach without making holes in the roof.
I personally think the sportwagon's look are enhanced by the rails especially in chrome. Saw one the other day at some traffic lights and it just looked the business. You must have noticed how good they look on the Audi avants and it just enhances the already beautiful looks of the Alfa.
PS When I get the wagon of my dreams I'll post a pic!