Not my field of expertise but I think the following might help, we had a similar issue a while back.
Navigate to the directory where your current key file is located -
openssl req -new -key your_existing_key.key -out your_new_san_file_name.csr -config your_config_file_name.cnf
Set all your information in pop-ups that follows.
Open your config file and add the SAN's for your certificate under the alt_names section.
Submit the CSR to obtain your SAN.