OpenPGP is typically used with asymmetric encryption (key pairs) but the standard contains a definition for symmetric (but still two level) encryption. With gpg this is done by using --symmetric instead of --encrypt.
KMail, Thunderbird/Enigmail and more clients can already decrypt emails which have been encrypted symmetrically. This can be tested with this file:
I suggest to add the capability to symmetrically encrypt mails to KMail. Symmetric encryption can be more easily used than asymmetric encryption if none has been prepared yet. For symmetric encryption the necessary software just has to be installed but not configured. People are familiar with passwords but not with key pairs. In the case of KMail nothing would have to be done as GnuPG is always installed. It doesn't make any sense at all to force the users to use something different from OpenPGP when they want to use symmetric encryption.
This is not supposed to be an alternative to regular OpenPGP usage but an alternative for those who are not willing or not capable to prepare for the use of key pairs in that moment. Most people would not use this feature permanently. If they start encrypting emails regularly they will probably switch to typical OpenPGP usage.
This feature should be very easy to implement.
There has been a long discussion on the GnuPG mailing list: