Jquery: Add an item to combobox after Ajax Query
===

I can't manage to find the solution to my problem. Thx a lot for your help.
I got the combobox example given with JqueryUI.

My source is a Json Ajax and all this works well. But I Want to ADD an item to my combobox (i want to append it to Ajax items) manually which say "ADD A NEW ITEM" and when i click on it, my modal form becomes visible.

I put in my code some commentaries where i need help to explain more my problem.

     $('#combobox_scenario').combobox({
        source: function( request, response ) {
			$.ajax({
				url: "?module=gestionApplication&action=getListeScenarios&option="+encodeURI($(".ui-combobox:first input").val()),
				dataType: "json",
				success: function( data ) {
					response( $.map( data, function( item ) {
						return {
							label: item.test,
							value: item.test,
							option: this
						}
					}));
					//I want to add an item Here !
					//response.add({label: 'Ajouter un Scénario',value: 'Ajouter un Scénario',option : this});
					console.log(response);
				}
			});
		},
		selected: function(event, ui) {
			var scenario = ui.item2;
			//if (scenario = "This Item added manually") 
				//MyForm.show()
			//else
				$.ajax({
					url: "?module=gestionApplication&action=getScenario&option="+encodeURI(scenario),
					dataType: "json",
					success: function( data ) {
						data = data[0];
						$("#scenario").show();
						$("#scenario").html('<hr><h2>Informations sur le scénario '+data.TEST+'</h2><p>Description : '+data.DESCRIPTION+'</p>'+'<p>Application : '+data.APPLICATION+'</p>');
					}
				});
			//EndIf
		}
	});

Thx a lot for your help !