If you want to add all your items to the promo you should create a hidden category (https://support.ecwid.com/hc/en-us/articles/207806725-Hidden-products-and-categories#creating-hidden-categories).
To create a hidden category:
From your Ecwid admin, go to Catalog → Categories.
Click + Add Root Category to create a new category (e.g., “Special products”).
Disable this category:
◄ Click +Add Subcategory to create a new hidden category
◄ Drag and drop the existing category that you want to make hidden into your root disabled category (this way, it you’ll turn it into a subcategory):
Make sure the subcategory is enabled.