Adding a random signature, just like the daily signature, is pretty straight forward. Open KMail by pressing Alt+F2. Enter kmail in the text box and then press Run. Once inside KMail, go to Settings → Configure KMail.... In the Identities selection, highlight the identity to add the signature to and then press the Modify... button on the right-hand side of the window. Select the Signature tab. Check the Enable signature box. From the Obtain signature text from drop down, select Output of Command. In the Specify command text box, enter ksig --random and then press the OK button.