SKULabs imports a WooCommerce product's barcode from a custom field named upc, gtin or isbn. A product attribute with one of those names also works. If a product has more than one, SKULabs uses the ISBN first, then the GTIN, then the UPC.
An easy way to add these custom fields is the free Advanced Custom Fields plugin for WordPress. The Pro version is not needed.
How to add WooCommerce barcodes with Advanced Custom Fields
1. In WordPress, go to Plugins > Add New, search for Advanced Custom Fields, then install and activate it.
Advanced Custom Fields in the WordPress plugin search.
2. Create a field group with a text field for the barcode. Set the field Name to upc, gtin or isbn. Create only the fields you need. Under Location, set the rule to Post Type is equal to Product so the fields show on the product edit page.
A field group with upc and gtin fields, shown on products.
3. Open each product in WooCommerce, fill in the new barcode field and update the product.
The barcode fields on the WooCommerce product edit page.
4. Run a listings sync in SKULabs. See How to run listings sync for your stores.
Good to know
New items that SKULabs creates from these listings get the barcode automatically. For items that already exist in SKULabs, check the item and add the barcode there if it is missing.
Custom fields are read from the main product. For variable products, add the barcode to each variation's item in SKULabs.