IPTables comes with many flags, options and target extensions. To keep the interface simple and easy-to-use, ERWALL only cosists some of them. However, restricting ERWALL users to a small set of options may make impossible to define some specific rules. You can use the custom field of rule creation form, in such cases. You can specify any option in this field and it will be attached to the commands that will run on your server.
Refer to the iptables manual page to see the whole list of possible options and target extensions.