Do Shopify products have to be uploaded to Stock Sync before I can use Stock Sync to update/manage them?
No, you do not have to upload the product using Stock Sync to update it. Stock Sync is able to update existing products that are in the store.
To update existing products in store, you can refer here.
Or you can choose to add new products to store, you can refer here.
Stock Sync updates products that were added to the store from any method. We do not restrict the updates based on add by Stock Sync only