This code add one or many items to GtkCombo object named combo

Just add one item like this

 listitem = gtk.ListItem("String 1")
 combo.list.append_items([Listitem])
 listitem.show()

Or if you like you can add two:

 listitem_a = gtk.ListItem("String 2")
 listitem_b = gtk.ListItem("String 3")
 combo.list.append_items([listitem_a, listitem_b])
 listitem_a.show()
 listitem_b.show()