howto modify actions performed by gnome's shutdown button

[Evuraan]

Ubuntu 9.10 desktop wont shutdown OK if I choose shutdown from gnome
(from upper right corner of the gnome screen) , I often have to
physically power down the sytstem.  I want to modify the actions
performed when I choose shutdown, for instance, I want to add

echo b  > /proc/sysrq-trigger

to the events triggered by the shutdown button.

How can I modify the shutdown button's behaviour? any help wud be much
appreciated, thx.