HostBill makes use of PHP PDO for database operations, and as such you can configure it to use SSL when connecting to PDO.
In your HostBill's includes/config.php file add following lines to bottom of file contents (ensure to specify valid cert/key/ca paths):
//...
//use below to specify additional options passed to PDO:
$config['PDO.options']=[
PDO::MYSQL_ATTR_SSL_KEY =>'/etc/mysql/ssl/client-key.pem',
PDO::MYSQL_ATTR_SSL_CERT =>'/etc/mysql/ssl/client-cert.pem',
PDO::MYSQL_ATTR_SSL_CA =>'/etc/mysql/ssl/ca-cert.pem',
//PDO::MYSQL_ATTR_SSL_VERIFY_SERVER_CERT => false //to disable server cert verification
];