I won't say absolutely you can't add rails - but the expense and difficulty doing it makes it really impractical. The 955/957 series cars came with the needed mounting points in the roof, even if the car didn't have roof-rails. The 958 series didn't - and adding them requires dropping the roof-headliner, and welding brackets to the roof structure, which destroys the paint on the exterior, then drilling holes through the roof for the bolts for the rails. If you mess up the spacing even the tiniest bit things aren't going to fit. I've heard of one person who did this, and they said they'd never do it again since it turned into a massive expensive job.
I'd say look to Thule.