So maybe configuring the email as imap/pop over ssl first will allow you to import the certificate.
We maintain a list of certificates for many popular mail servers requiring SSL access (e.g. Gmail, Yahoo!) allowing users to connect to them using this service. However, if your mail server requires SSL access and the certificate for your specific mail server is unavailable on our system, you will not be able to connect to your email account.
I agree, from what you've quoted it looks like custom certificates are not supported.