These instructions relate to KMail 1.12.4 (that is KDE 4.3.4).

When choosing “Forward: As Attachment …” KMail will create a new message with the message to forward attached, and also attachments of that message included. But you cannot chose to use a template from your template folder then (with preconfigured text and recipioents).

1. Save the e-mail to forward as a local file with the .mbox extension. This is just a text file with the mail’s source text.
2. Create a new mail from the template in your KMail templates folder.
3. Choose “Attach: Attach File …” and attach the .mbox file you just saved.
4. Remove the .mbox extension from the attachment.
5. Set the MIME type of the attachment to “message/rfc822″, which is what KMail
   uses when you choose “Forward: As Attachment …” to forward a message. Else, KMail will not honor the “suggest automatic display” setting for this attachment.

At least KMail can open such an attachment as an e-mail, also including
attachments to that attachment (like image files). It is unclear
currently how this displays in other e-mail programs.