Can I add another domain later, after the SSL certificate has been issued and activated?
Yes, you can add more SANs to your SSL certificate any time after issuance, provided the existing SSL certificate is listed below:
EV Multi-Domain SSL
In order to add SANs to a certificate, one will need to perform a reissue from within their Namecheap account. It is not possible to upgrade any other SSL certificate type to SAN.
If you need to purchase the additional slots for the domains, please use the option Buy more domains seats shown on the screenshot below.
All additional domains need to be entered in the corresponding fields Add-on domains.
You can, however, request cancellation and refund of the existing certificate, purchase the needed SSL certificate, and activate it with the appropriate number of SANs. Comodo (now Sectigo) certificates can be refunded within 15 days after issuance date.