As I understand, and as it was stated here, when External DB is the method chosen for user authentication, it is not possible to allow users to change their own password, unless you define password format as "select internal" for all users, as explained in this post.
Can anybody confirm this?
Is it impossible creating users from an External DB with a given password, and then allow them to change it?
Moodle can't manage the changing of passwords in the external database, there has to be a web page available for this. If such a web page exists – this could be created if it doesn't exist – then you can put its address in the Password-change URL (auth_db | changepasswordurl) field. Then when users who authenticate using the external database select the Change password link on their Preferences page they are taken to that URL instead.