Is it possible in a FXML-file to include another FXML-file that describes for example a menubar?
Yes it is. For example:
FXMLLoader loader =
new FXMLLoader(this.getClass().getResource("YourFXML.fxml"));
tabPane.getTabs().get(0).setContent((Node) loader.load());
With the setContent Method I could set a FXML as the Content of this tab. In other panes you could do the same. An anchorpane could do:
anchorpane.getChildren().add((Node) loader.load());
Hope that helps you ;)