To add Tabs without programming using the JavaFX Scene Builder:
After the TabPane has been added, select a Tab in the Hierachy, then right mouseclick, select duplicate from the popupmenu. A new Tab is added.
Alternatively, open the layout fxml file (from the files folder) in a texteditor, copy & paste a tab section, like <Tab text="Tab 1"> ... </Tab> and make further changes required.