This question is outside of the scope of Category Previews, so I moved it to its own topic.
You can add content above the boxed category list using a plugin outlet. For example you could add the following to the “Head” section of a theme/component:
<script type="text/x-handlebars" data-template-name="/connectors/above-discovery-categories/custom-category-section">
This text appears above the category list.
</script>
And that would produce something like this:
You can obviously add your own HTML/CSS to make it look nice.