Can I add more years to the SSL certificate I bought?

Unfortunately, no. Only if SSL certificates are renewed within 30 days of expiration, those days left are added. 

If the certificate is renewed earlier, there is no benefit and the certificate will be issued as if it is issued on that date. 

If the cert is going to expire within 30 days, you will get an email with a renewal link, or you can click on the renew link that will appear next to the SSL certificate in your account to get it renewed.

When renewing or ordering a new certificate, multiple years can be applied.