I just went on their the manufacturer's website (LeSaffre) but the section on manufacturing methods is under construction. But I can assure you this is right. I, personally, have emailed with the director of the yeast division. They add the b vitamins in after fermentation but before pastuerization. I'm sure there are many places on the web you can use to verify this information, but I wanted to have you read it directly from the company's website. Wait, I just found a decent link:
Nutritional Yeast Division - Lesaffre Yeast Corporation
You will see links in the middle of the page on the right -- click on the VEgetarian support formula. At the bottom you can see the ingredients in the Vegetarian support formula (includes many of the b vitamins including b-12). Without this supplementation, there is no b-12 in the nutritional yeast. You can see this by clicking on the link to NBC600 on the right of the screen I gave you. This has no supplemented B-12 in the ingredients, and 0% of B-12 in the product.
Hope this helps!
Joey