How to add a field to the contact form in Drupal 7?
This module http://drupal.org/project/contact_field will do what you want and I have know about it since D6, but the last time I try it in D7 it was buggy. 

So far the best alternative I have found that works all the time like a charm is the webform module http://drupal.org/project/webform as you will be able to add or remove fields as need it and the email support works well and you also get the bonus of storing all the contacts attempts in case your email server or service was down for any reason.