If you want to create a filter to sort a message into a new folder, you can create a new folder in the folder selection dialog. However, that dialog doesn't see the folder (even though it is created on disk) until KMail is restarted. Reproducible: Always
This bug has only been reported for versions before 4.14, which have been unsupported for at least two years now. Can anyone tell if this bug still present? If noone confirms this bug for a Framework-based version of kmail2 (version 5.0 or later, as part of KDE Applications 15.12 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.