I'm not a GTK+ expert (I use Qt), but I found this in the documentation:
ToolItem is a subclass of Bin which is a subclass of Container. Container has an add() method to add a child. So I would try this:
item = gtk.ToolItem()
Note that it is not necessary to prepend "self." to all variables which hold a widget, you only need it for those that you will reference later from other methods.