To import the email conversation to Workable, locate the candidate in Workable. At the bottom of the candidate's Timeline you will see an option to import emails between you and the candidate. Click the link to import.
If your account does not have an email integration enabled, then you should begin all email correspondence through Workable.
After you start the email thread in Workable, you will be CC'd on the candidate's reply in your regular email. From that point you can reply via regular email and the communication will sync to Workable automatically. Email communication started outside of Workable is not able to sync back to Workable.