Add CC to email campaigns

Currently, I can specify "To", "From", and "BCC", but not "CC" in the transactional email template. We'd like our campaign to send an email a third party with our client CC'd. This would enable the third party--who has an existing relationship with our client--to see that our client is aware of the email. When we put both the third party and our client as a list in the "To" recipient, the email is not sent.