You can add or change the SANs in your order by simply performing the “Re-Issue” process of your certificate, during which a CSR will be requested and GeoTrust provides you a new certificate file.

You can start the reissue process from your client area in DomainRegister.international; after the log-in, go to the administrative page of the SSL certificate you need to reissue and click on the "reissue" button. 

If you have already used all your allotted existing SANs, You will need to purchase additional SANs before starting the reissue process: raise an assistance ticket for further details.