It is possible, but can be a little complex depending on what you’re trying to do. The way we most often approach this is by overriding the template for the default category page in a theme (category boxes with subcategories in your case)… and then setting that as the homepage (controlled by top_menu in Discourse’s admin settings). That template is https://github.com/discourse/discourse/blob/master/app/assets/javascripts/discourse/app/templates/components/categories-boxes.hbs
Template overrides are covered in the advanced section of Beginner's guide to developing Discourse Themes