How to add a custom element to grid elements (visual composer)

Hi i want to add a custom block to visual composer ---> grid elements.
By default you can add on your grid elements: title, featured image, post date ecc but not the categories of the post. I found this article who explain in details what code to implement to see new featured but there is no information about the file to modifying..

http://ift.tt/1HY8SPZ

Anyone can help me?

Thanks in advance

via Chebli Mohamed