[exim] Private header / tag

HI all

Would it be possible / allowed to add a private header / tag in the
creation of an email.

For example, below is a list of tags / header that is used for a
creation of an email

From: Name Surname <xyz@???>
To: exim-users@???
Content-Type: text/plain
Organization: XM Solutions
Date: Thu, 19 May 2005 09:28:26 -0700
Message-Id: <1116520106.10073.6.abc@???>
Mime-Version: 1.0
X-Mailer: Evolution 2.0.1
Content-Transfer-Encoding: 7bit
X-Spam-Score: -1.3 (-)
Subject: [exim] Header error message
X-BeenThere: exim-users@???
X-Mailman-Version: 2.1.5
Precedence: list

I would like to know if I can add to this list of tags / header.

Any tips and or advice would be greatly appreciated.

Kind Regards
Brent Clark