Currently you cannot add an email address to an endpoint after it has been paired. You have to do a factory reset and specify an email address during the pairing process.
This is troublesome when you have a number of endpoints already paired and installed at interstate branches across the country. The reason I'd like to add an email address is so the endpoints can be dialled via the email address as a SIP URI. I would need to also update my DNS records so that @company.com would work (Configuring SRV records).