What I want to happen is for Hunt group 450 to overflow to a mailbox that will light a light on two other extensions

can I temporarily hook a phone up to a unused station port and then forward that ext to VM then overflow 450 to that or is their a better way ?

Just overflow 450 to the voice-mail group (440) and set the VMID in F30 B17 to the mailbox you want calls forwarded to. If needed, you can use an "unused" mailbox and assign a separate button on the phones that need access to these messages.

I had something similar I needed to do yesterday.

The auto att went had an option for a general mailbox ex (ext 155 virtual and fwd to mailbox vmid 155 immediately). Then they had 4 ext in the office that wanted a button for that mailbox. (speed) (speed) 460 + (155) for the general mailbox in addition to their own vmail button.

I got it set up , worked great Thanks