How to create a definition for a metafield that already exists?

[Smeelah]

Hi,

I am using this metafield to hide my wholesale products (Namespace seo, key hidden, integer 1). It was added manually or with an app a couple years ago.

However, I now want it to be available directly on the product admin pages. I cannot seem to create a definition for it, as it is 'already taken' and I do not see it in the list of 'metafields without definition'. 

If I remove the metafield from all products, would I then be able to add it back in the shopify metafield section?

Thank you!