X-Cart forums - How do I add a hidden field for products?

I have the Firetank Feed Manager & I have just started to use it. I signed up with Shopzilla and went through their feed requirements & they require that you put one of their category ID numbers in for each of your products. I could use my category names, but I'm pretty sure that would dump all of my products into the Other category =). I know that it is possible to add an extra hidden field to the product listings, but how? I would like to put the shopzilla code on each product as I set it up & just add it to the feed. Please keep your instructions verrrry simple for me! Thanks!