Unfortunately, your Sales Channels e.g. Amazon, eBay, Etsy etc. can not be used as the Source.
Shoppingfeed can not export your listings from your channels/marketplaces into your Shopify (Bigcommerce, Magento etc) Admin at this time.
In simple words, Shoppingfeed helps you take the product data from your web store or any other source and push it over to the selected Sales Platforms like eBay, Amazon, Google Shopping, Etsy, Walmart, and many more in the required format.
If you used to create listings on your Sales channels/marketplaces initially and want them to be exported to your newly created online store, there are several services & apps that can help you with such a task.
Note that as soon as you have the same products in your online storefront and on your sales channels, Shoppingfeed will be able to take over those existing listings on those Sales Channels (SKUs should match though) as well as help you create new ones to bring more Buyers to you.