Well it all depends on how your spam solution is setup. I'm guessing the developer is piping email to spamassassin via the qmail-queue envir. var. If this is the case, then mailfront mailrules are applied before messages are sent to spamassassin.
Therefore, you only need to define a mailfront mailrule to have the rule applied before any spam scanning.
How you define a mailrule depends on what you are using to control this rules. Have you installed a contrib or did you set it up yourself?
Greg