Unfortunately, you can't customize the Reply/Forward headers without the help of QuoteFix or ReplyWithHeader.
To customize the actual headers when viewing an email message, I'll quote this MacFixIt article:
To alter the headers that Mail includes, you can go to the Viewing section of the Mail preferences, choose the Custom setting, and then add or remove any fields of the e-mail header that you wish to see. Unfortunately you need to enter the field names manually, and many times e-mail services will append their own header details to a message, especially for handling spam or other security features. Therefore if you are not sure what the header titles are then you will need look them up in the raw source code for any particular message.
To do this, select a message in your inbox and then press Option-Command-U or choose "Raw Source" from the View > Message menu. In the resulting window you will see all the message details in plain text. Header fields will be the text before colon characters with dashes in them in place of spaces, and may include items like "Delivered-to:" or "Return-Path:", or custom items like "X-Virus-Scanned:" for antivirus information.
When entering these details in the custom headers list, be sure to enter them as is (including the dashes), after which be sure that Custom is selected in the preferences and that the Details link is clicked in your e-mail messages.