Te reason you may want to add a corporate contact to your personal contacts is because it will then allow you to add them to groups to allow for easy conference calls.
To add a contact to your phone press the contacts button, as seen below. This will take you to the contacts page.
Press the right arrow twice to get to the Corporate Directory. Either search for a contact by typing their name or use the arrow keys to scroll through the list.
When you have found the correct person press the softkey +Contact this will allow you to add them to your personal contacts.
Within this screen you can change their group, give them an alias (nickname), and modify their ringtone. Once you are happy with the settings press the softkey done.
The corporate contact will now appear in your personal contacts.