Hi Nan_A,
If you take a backup of the controller, un-zip the tar.gz backup file. Navigate to the "Flash" / "CertMgr" / "ServerCert" folder. In this folder, find the file whose name is the same as the friendly name you gave to the certificate when it was previously imported into the appliance. Open the file in a text editor and copy the "Encrypted Private Key" section. You can then take this value and paste it into a .pem file that contains all of your necessary certificates (root, intermediate, etc.) and re-upload the merged/combined file, containing all certs and the key, back to the controller.