I want to add new attributes for my catalog in Sitecore Experience Commerce 9 update 2 but not sure how to add it using the Sitecore Experience platform. Moreover all the properties data need to be fetched and populated from an external API. What should be the approach to do so?
Have a look at this article for extending commerce entities (products, catalogs categories users etc.)with custom properties https://hachweb.wordpress.com/2018/07/15/sitecore-xc-9-0-2-walk-through-extending-product-definitions-with-custom-fields/ And this one for getting axcess to them within the engine https://hachweb.wordpress.com/2018/08/01/sitecore-xc-9-0-2-how-to-retrieve-composer-generated-templates-within-sitecore-engine/
Hope this helps you.