I am not sure I understand... What exactly do you want to modify in the Administrators (999/9999) Mailbox?
The Admin Mailbox should never be used for "normal" purposes.
The Operators Mailbox (0) can be used, although I prefer to make it a Transfer Only mailbox and give any "operator" a real mailbox.
BTW, you can't delete mailbox 0 or 999, they are there permanently. To reset them you will need to do a "To delete this installation and create a new one, Press 1" from the Administrators Mailbox.