Hello reedredux,
Thank you so much for using our theme AcmeBlog. As per your question, you need to customize the theme to obtain your requirements. According to the WordPress standards, we can only manage such section by category or page. In the free version, you can manage the slider either from recent posts or from category. If you want to customize the theme, you will get the theme codes inside acmethemes/hooks/slider-selection.php and customize the theme as your need.
Recently, the post displayed on the slider are recent posts.
If you need any paid customizer, you can request as well.
Regards
Acme Supports