If you have the passwords for the system, just go to that menu in programming and change the '8' directive to 'Record Prompts'. Save the change, dial into the menu you just changed and press '8' enter password and follow the prompts to start recording. When finished, change the prompt back to what it was.

You can also change the prompt from a mailbox if your COS is allowed to change system prompts. If I recall correctly, you need to sign into your voicemail box, press 8 for special functions, then enter the password. It should prompt you for the menu number you want to change.

Tip and ring, the work of the devil!