there is an easier way an also more secure than logician said:
you have to add a field to table post.
then make a few changes to editpost.php, 1 small addition to functions.php (getpostbitsection) and 2 new templates.
if you have made it right, it's really secure, because everything written at this field than can just be read by mods, and nothing of the text can be seen in the searchfunction (neither by normal users nor by admins)
thats the price of freedom