Currently, additional emails are only accessible to send to from the contact record. The "Email" action and "Notification" action in workflows will only send to the primary email in additional emails, even if you have specified a secondary email as the recipient in the "Notification" action.

The idea here would be to give users the ability, when setting up an "Email" action in a workflow, to select any of the 10 allowed emails from the additional emails field as the recipient (i.e. Email 1, Email 2, Email 3, etc), defaulting to a "skipped action" if an email does not exist in that spot. OR, allow the user to choose what happens if the email is not found in that spot ("skip or send to Email 1").