If you have an existing cert and key imported to LTM and renew the cert, it should use the existing key. I'd test this with a dummy cert/key first, but I think it should work like that. Else, like Nitass says, you can use openssl to do this on the CLI.
Note that in v11, not all of the cert/key files are stored in /config/ssl/. They're now under /config/filestore/files_d/Common_d/ with links for default.crt, default.key, ca-bundle.crt going back to /config/ssl/. And you must use the GUI or tmsh to import certs and keys into the filestore. Modifying files in the filestore and reloading the config doesn't work anymore...
Aaron
Nimbostratus
