To add your own SSL certificate, edit the JetBackup MC configuration file located here:
/usr/local/jetapps/etc/jetlighttpd/conf.d/jetbackupmc.conf
Change the following line on the config file:
ssl.pemfile = "/usr/local/jetapps/etc/ssl/private/jetbackupmc.pem"
And specify the path to your SSL certificate.
ssl.pemfile = "{SSL_CERTIFICATE_PATH}"