BEAR - WooCommerce Bulk Editor and Products Manager Professional

Is it possible to add new terms to WooCommerce product attributes in bulk?

Yes! To add new terms to WooCommerce product attributes in bulk, apply operation 'Attaching' using Variations Advanced Bulk Operations. It attaches a new attribute, with a term for every existing variation, to variable products that already have their other attributes set up:
  1. Filter variable products in the Filters tab, or tick them on the page.
  2. Open Variations Advanced Bulk Operations from the tools panel and go to the Attaching tab.
  3. In the top drop-down choose the attribute to attach; after its terms load, pick a default term or leave it.
  4. Enter the ID of any one product that has the same set of attributes as the products you selected, and press Get existed variations.
  5. For each existing variation choose the new attribute's term, or ignore to leave that variation alone.
  6. Check the table twice, press Attaching and wait for the operation to finish.
If the attribute is already on the product but not marked Used for variations, attaching does nothing; run the Visibility operation of the same tool first. These operations are not covered by History, so back up the database before running them on a large set.