new_choices = 'one', 'two', 'three'
for choice in new_choices:
    network_select['menu'].add_command(label=choice, command=tk._setit(var,
        choice))
network_select = tk.OptionMenu(root, var, *choices)
network_select.grid()
tk.Button(root, text='Refresh', command=refresh).grid()
root.mainloop()
Changing the options of a OptionMenu when clicking a Button