What version of Pega are you using? If you are using Pega 7, and Email Listener to manage your work objects, you can select Save original email (in EML format) option on the Process tab of Email Listener. When this is enabled, the inbound email gets attached to the work object.
Yes you can modify the subject.The subject will be saved in the properties which you are using in the request.In my case i am using the ".pyInboundEmail.pySubject".Before attaching the attachement you can modify the subject and save it on the page with the class of Data-Corr-Email and use link-objects method to add the email attachment to the WO.
If work object is already created then apart from the configuration specified by Praneeth, you also need to get the attachment (original email) from pyAttachmentPage (page created as part of EmailListener processing) and link it to work object.