This should work in newer versions:

touch /etc/httpd/conf.d/iris.conf

ISC_PACKAGE_CSPSYSTEM_PASSWORD="my_password" ISC_PACKAGE_PLATFORM="lnxrh8x64" ISC_PACKAGE_HOSTNAME="localhost" ISC_PACKAGE_SUPERSERVER_PORT="1972" ISC_PACKAGE_INSTANCENAME="IRIS" ISC_PACKAGE_INITIAL_SECURITY="Normal" ISC_PACKAGE_WEB_CONFIGURE="Y" ISC_PACKAGE_WEB_APACHE_CONF="/etc/httpd/conf.d/iris.conf" ISC_PACKAGE_WEB_GATEWAY_DIR="/opt/webgateway" ./GatewayInstall quiet

Is it possible to set this using iris merge functionality?