Ok, I’ve found a solution. For others: GTINs (Global Trade Item Numbers) including UPC, EAN, and ISBN are globally recognized unique identifiers for products. Woocommerce is the only shop system which doesn’t provide a field for that – hard to believe. It is getting more complicated when using variable products and even more complicated when publishing a Google product feed for example.
Solution: https://www.youtube.com/watch?v=5ncHUuSJK0Y
TL;DR
1. Install WC Field Factory
2. Create dynamic fields (EAN, UPC etc.)
3. Assign them in your product feed
Thread can be marked as solved.