If the email address is already associated with a contact, Freshdesk displays a banner "Email has already been taken."
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Click on 'View User' in the banner to go to the Contact Profile page.
Click on the "Convert to agent" option.
Choose whether the agent should be full-time or occasional, and click on Convert to create an agent from the contact profile.
Now, navigate to Admin > Team > Agents and click on Edit next to the name of this agent.
Enter the Signature of the agent if required.