Take a look into the ssl directory of the website where you created the signed certificate. There you find a key and a crt file. Kopy the content of the key file into the ispconfig key file, the content of the crt file into the ispconfig crt file. Then create a new file for the bundle certificate in the ispconfig ssl directory and insert the bundle certificate(s) there.
Then add a line a line like:
into the ispconfig vhost file as ispconfig as no support for bundles builtin and then restart apache.