Yes, it is possible to increase the SAN slot count for an existing multi-domain SSL certificate. After upgrading your SSL SAN slot count you can re-issue your certificate including the additional SAN slots. The upgrade price per SAN slot equals the price for an additional SAN slot for a newly ordered certificate, regardless of the remaining SSL certificate lifetime. A SAN upgrade does NOT increase the remaining lifetime of the underlying certificate. It just increases the count of available SAN slots.
If you want to order a SAN upgrade, please open a support ticket and specify the exact SAN upgrade count as well as preferred payment method in case it differs from the initial SSL order.
