It’s not possible out of the box I’m afraid. The menucard is loaded dynamicly with all the categories, so adding a message box to the bottom of one of them is hard to do.
However, you can add text to the menucard categories. The difference though is that it’ll be just under the category titles on your menucard. You can achieve this by going to Posts -> Categories, and give one of your categories a description.
Hope this helps 🙂