Hmmmm.... I am sitting in front of Outlook 2003 (on my work PC) and I am unable to change the title of a received email. Is this a setting? While this may not be possible on iOS, your best bet is to create smart filters or use a smart filter service to parse your emails so you know everything in a folder is an "order".
I'm running 2010, although it worked on previous versions too. You just open the received email, and either highlight and overwrite the subject, or click next to it to append text.