Edit subject line of email in inbox in Apple Mail
While technically it's possible, it's not trivial. You would have to write an applescript to search/replace the contents of your subject field and then use apple mail filters and run applescript based on your rule.

This is commonly done in the unix world via procmail, for example when spamassassin detects spam, procmail appends SPAM to the email subject message.

it might be better to set up rules based on subject to filter your mail into separate boxes for organization.