Unfortunately, I am not aware of any possibility how to achieve this with the merge settings which are available today.
Can you explain your use case a little bit? Do your suppliers provide data classified by multiple systems? Maybe you can adjust your import mappings to not import those attributes in the first place if they are not relevant to you. Or export connected features only, so that the free attributes don't cause any issue?
Thank you for your response.
Our suppliers provides data for multipe classification systems, including their own. And although we don't want to use this data (attributes + values) or classification (to groups) in our master catalog, we do want to store it in the supplier catalog.
Does this help?