For future reference, I will post a possible solution for this problem, but nevertheless, I think this is a lack of a feature necessary in these days: have a module that allows to choose a specific product to offer Free Shipping and reflect it in The Product Feeds
I found a way to hack this by messing with weights. I will explain what we did for our customers and get it to work.
1 - create a new interval on the Default Shipping Carrier with a absurd weight (for example 10 000 Kg to 100 000 Kg)
2 - put 0 as the Shipping Cost for that interval in the zone(s) you want to offer shipping (specially, on the default zone so that the feed creator pick up this 0 cost shipping at this weight interval)
3 - assign that absurd weight on the product you want to offer Free Shipping (in this case, 10 000Kg)
4 - You're done!
In my opinion, this is a hack that prevents you to buy all the Free shipping modules that announces that can configure Free Shipping on Specific Products. After I bought several of them, I realized that most of them were only a replica of what Prestashop already has (Cart Rules) which have a poorer UX (when you add a product that you want to have Free Shipping to the cart, by using Cart Rules it doesn't show "Free Shipping" on the popup window, which is a visual mistake IMO) and others had a more correct approach to this issue in terms of UX but the Feed Generators couldn't communicate with these third party modules making the feed incorrect.