You can't import one until you create one, Create the self signed, reboot and the option to import will appear.
You will find all the latest firmware and downloads HERE.
I would recommend contacting the Support Department for more information on your question. The support options for your device are listed HERE.
Security | Machine Digital Certificate Management only has the options to create a self-signed certificate or certificate signing request. There is no import option.
Security | IEEE 802.1X only shows EAP-MD5, EAP-MS-CHAPv2, and PEAP / MS-CHAPv2 for authentication methods. We use EAP-TLS on the 6515 and 8000s, which allow you to choose an imported device certificate to present to the authentication server.
The Last Successful Upgrade shows Version 1.200.35. Am I missing something or is there newer software that enables these features on the C60