Is there any way to change the off hook "Dial station number or access code" message.

I've been asked to change it to "Dial 9 for an outside line."

Nope.

I can't think of any system messages that can be changed, unless you can figure out how to hack the CIX system software (which would not be recommended).

You can use a label machine and put a sticker on the phone that says "Dial 9 for an outside line."