Please do what xaver suggested and run a ispconfig update where you select to create new ssl certs. If your ispconfig is up to date, then download the ispconfig tar.gz manually from ispconfig.org, unpack it and run the update.php script.
I always have this question in my mind. Either updating or using the step from the link above I need to create a new SSL Request cert. So in other word my current provided cert is useless because I need to paste the new cert request to my cert provider and pasting the provided cert to ispconfig. Am I correct?
You can use a existing cert as well, but only after you did the step with the update that xaver described. Afterwards you can replace the key and crt file in /usr/local/ispconfig/interface/ssl/ directory with your existing cert and key.