The product customizer has an importing products tool. It makes you able to add multiple products at the same time to the custom products list, no need to click one by one.
On the Shop admin, open 'Products > All Products'
Select any products you want to add to the product customizer app.
After products select, click “bulk action” button. The submenus will appear, click 'import to Customify' menu to start the products importing process.