While creating a menu for your Magento 2 website, you may want to import your existing categories to save time. Ninja Menus, a powerful mega menu extension for Magento 2 does support this function. Below is a detailed tutorial on this.
https://www.magezon.com/magento-2-mega-menu.html

1. Open the menu edit page.
2. In the Menu Builder section, click the import icon on the top bar.
3. A popup will appear where you can choose the categories you want to import by searching in the Category field. You can also import the subcategories of the selected category instead by slide Import Subcategories button to Yes.

Remember to save after finishing. The categories (or subcategories of a category) will displayed on the highest menu level.