Comment 15 for bug 156288

As a workaround you can set "Only attach link to message" in the KOrganizer "Default email attachment method" option.
Then you can do the following:
- drop the email to Calendar icon
- copy the link to email (something like kmail:14327) inserted in the attachments of the event, then click "Cancel".
- drag again the email on the "To-Do list" icon, add an attachment with the plus icon and paste the link to the email in the URL.

Quite uncomfortable, but indeed works.