If the event you need on a case is not already included in its standard events or iterations, you can still insert it manually.
Open Manage Events by clicking on the "Manage" button on the Events tab.
On the Edit Menu drop-down, select Add Event.
In the Add Event form that opens up, go to Add Single Event and select the event that you want to add to the case.
In Target Date Properties, make sure that your new event is in the correct spot by checking whether the event name in From Previous Event is the event you want your new one to follow. If it's not, you can select from any of the events already on the case.