Though KUMail (Gmail) doesn't have such a function, accessing a certain URL like https://mail.google.com/mail/?cc=CcAddress&bcc=BccAddress&view=cm will open a composing window with Cc and Bcc set by default.

When you use this URL, replace CcAddress/BccAddress with an actual Cc/Bcc address and bookmark it on your browser. If you need Cc or Bcc alone, leave out &bcc=BccAddress or cc=CcAddress.