You can copy an address card from the global address book to one of your own address books, and edit it there.
In my opinion all you need is there already.
Or what functionality do you need in addition?

No Christian, maybe I didn't describe the feature request in enough detail. But I think you are forgetting the special features of the global address book here.

But let me give it a try. I am sure we agree that the following applies:

• A shared address book from LDAP/SQL is not writable. This is stated in the documentation of the parameter isAddressBook.
• To find users and use the Free/Busy display, isAddressBook=YES must be set for the data source.

So we have to set the parameter isAddressBook=YES for our authentication source (is equal to user source).

If for reasons (e.g. EU-GDPR) only the mandatory data should be included in the authentication source, the result is a incomplete address book. Because it contains only username and e-mail address.

As a workaround, it would be possible (as you suggested?) to have an additional address book maintained by a user, who then shares this address book with all other users and also activates the option "Subscribe User". The latter only works for existing users and not for "Any authenticated User". This in turn means a lot of effort when setting up a new user. But okay, it would be one way.

But with this workaround, the incomplete global address book is still there and we deprive ourselves of such nice features like the already automatically shared global address book (on all channels like CardDAV, EAS, etc.).

So from our point of view it would be great if the data from the authentication source could be enriched directly in SOGo. Because the UI for this is available and great implemented. The authentication source can remain read-only as before, new data simply ends up in the corresponding SOGo table in SQL.