You seem to be talking about the Directory extensions feature, as detailed here: https://zcusa.951200.xyz/en-us/azure/active-directory/hybrid/how-to-connect-sync-feature-directory-extensions
First of all, this is needed only for attributes that are not present in the Azure AD schema, if you simply want to add an attribute you've previously excluded, use the "Azure AD app and attribute filtering" feature instead. But if you do want to add an attribute that is not part of the schema, you simply need to add it to the list, no need to add any "existing" ones. Do make sure your expectations are set correctly, as such attributes cannot be "consumed" directly by any of the first-party apps (Office 365 as the prime example). Instead, you leverage them directly via Graph API calls in your apps. Refer to the document above for more details.