The answer is yes, it is available as a dealer-installed option. It will be cheaper to buy the car with the rails already mounted (same is true of tow hitch) because as a dealer-installed option, you'll have to pay dealer labor.
Also, up until recently, dealers weren't able to get the rails as a part. Apparently the manufacturer who makes them for Infiniti couldn't crank out enough to keep pace with new car production and therefore wasn't releasing any into the dealer supply stream. But that is now changing. I ordered rails for a customer yesterday and was told we should see them within a few weeks. I'll believe that when they actually show up, but the story I'd been given until now was not to even bother ordering them - so this is an improvement.
If you want real, elevated, metal roof rails that can actually do something, you might want to wait to see if the Pathfinder's will fit the roof seam and line of the JX. With nearly everything else functional common with the Pathfinder, I am really curious about that. The JX roof rails are just cosmetic plastic bumps with pop-off plastic caps in the middle to expose connection points for the too narrowly spaced cross bars.
Nissan Canada has Thule attachments available for the JX roof rack. There is a Canadian Infiniti dealer selling the options on ebay for a reasonable price. You can also see them on Nissan Canada's site.