This is a SIP phone, it works a little differently... To make changes to the keys you would have to change it's configuration files if it isn't something you can do via the web interface. Polycom has reference documents on their site for this, they are quite a read. I have never done this in the SS IP5k phones, but we make changes to other SoundPoint devices all the time on our hosts SIP platform. This would also require a tftp/http server for the phone to grab it's configs from, I don't know for sure you could load the profiles on the 5000 (it is possible on the 3300).
Perhaps it would be easier just to set it as a House Phone and set the number to 8? Then any time a call is sent, it goes directly out.