How to concatenate old mail content and latest email reply received

Use case: There is a requirement, where on receiving an email, an interaction should be created. The mail body text should be displayed on 'description' field.  And if we get reply to the mail, the description should be update.

I have created 2 inbound action, one for New type, other for Reply type. 

Ques: How to showcase mail that was send for creating an interaction and on the same field, if any reply comes, that should also be displayed. Basically concatenate both emails.