If its a new install, just install Bind instead of mydns.
If you do this on a existing setup, then unsinstall mydns, install bind and run a ispconfig update. Choose to reconfigure services during update. Bind does not know about any domains that wre created in mydns before you installed bind. So if you have existing DNS records, you will have to change a setting in every zone and click on save to get it written to the bind config files.