My first impression is that it is not inherently possible... You can create business attributes with email address sub-types or CC container behavior and try to work them into a mapping, but I don't see a way for you to alter the Message Attributes that are provided for mapping. Only From/Subject/Body/Attachment are provided, and it seems to me as though this is something that's hard-coded.
Hope I'm wrong.
I am trying to find a way to do this now. We have set up an inbound mailbox to create tickets, but people (unfortunately) like to include cc's. It would be great to have a way to do this.