Hi Michal,
might also consider to have a look into these topics:
https://docs.shopware.com/en/shopware-platform-dev-en/how-to/add-admin-new-column
https://docs.shopware.com/en/shopware-platform-dev-en/how-to/add-admin-new-field?category=shopware-platform-dev-en/how-to
It is not 100% for your case, but it should also work for your case aswell.
Kind regards,
Tim