Currently, we have no plan to integrate it. But feel free to make a feature request in our issue tracker at git.ispconfig.org.
Actually, I think it is possible to just set acme.sh to use ZeroSSL. But this is not officially supported.
Actually acme tried to use Zerossl after update which caused me some issues on a couple of domains which are solved after selecting on all webservers to use let's encrypt as default.
Which ISPConfig version do you use? If I remember correctly, we choose LE as SSL cert provider by default in the latest versions for acme.sh.
When ISPConfig is installed or updated, we set acme.sh to use le with command options: --set-default-ca --server letsencrypt so it should be using le by default.
