Adding items to custom.cf for Spamassassin

I am trying to add a custom header to Spamassassin. I am editing the custom.cf file in /etc/mail/spamassassin but it doesn't seem to be working. The line I am adding is:

add_header all Level X-SPAM-_STARS(X)_

We have a current mail filter that adds this and is what our mail server looks for when moving items to junk. Is this something that should work and I am doing it wrong or can this not be done?