Unfortunately it is not possible to compose an email message from with the CS app and attach an existing email to it. As well there is no possibility to save an email within CS app and attach it to another communication. Using MS Outlook and the Dynamics plug-in is no solution here, as agents are supposed to work in ONE environment and not multiple ones, so ideally similar functionalities should be provided when composing an email in CS app as there is for Outlook to generate a better user experience.