It would be nice if there was an add-on that would sync your DNS changes to your cloudflare DNS. I found something that might do the trick, but now a hook into ISPCONFIG needs to be put in place to kick off the script when the dns records are updated. https://github.com/debitoor/cloudflare-zone
I do not think you mean to make CF your secondary nameservers as that is not possible last time I checked. If you mean to import dns info from ISPConfig, CF is smart enough to get them all if you migrate to it. So, I am not really sure what you mean but unless I am wrong, updating dns in CloudFlare does not need ISPConfig script whatsoever but you only need to run a proper command from the CLI or use a script. Anyway, that depends on what you really mean.
cloudflare replaces your name servers for a particular domain. So if you decided to use it, you always have to do to their website to make changes. It would be nice to be able to select enable a cloudflare sync setting. So if you add DKIM keys to a zone, you don't have to go through the trouble of exporting the zone file from the server and importing it into cloudflare every time.
I transfer all my domains to CF more than two years ago and remember it detected and imported all dns settings properly, however, that may be because my domains dns info are not so complicated. That's said, I think the addon could be useful for such a migration but I do not think ISPConfig developers would do that since the main concern should be how to import dns settings / info to ISPConfig dns server rather than vice versa. So I think the best would be, CF developers or any individuals with knowledge and up to that challenge with or without such a need should develop one and share it publicly with others like in the link you provided earlier.
The function in general would be useful, to maintain DNS records in numerous/any external DNS service, particularly for single server systems without multiple ISPConfig DNS servers available. One nicely done, working example might be appropriate, upon which community-maintained versions for different DNS providers would be based. Somewhat related, I have tossed around the idea of creating a similar plugin to interface with a domain registrar to add DS records, in order to ease dnssec deployment. But that's far down the priority list now, so not likely to happen.
