How to associate a contact with an account if the contact has been created before the account?
From an Account's Details page, you can add only new contacts and associate them to the account.
If you want to associate an already existing contact to an account, you need to update the Account Name field for the contact. Once you update the account name, the contact will be automatically listed under the respective account.
Please note that you cannot update this field by using the Mass Update feature. You can update only one record at a time.