wineclo we already have an open-source cPanel → OpenPanel importer available here:
https://github.com/stefanpejcic/cPanel-to-OpenPanel
There’s also a simpler script for CyberPanel:
https://github.com/stefanpejcic/CyberPanel-to-OpenPanel
For ISPmanager, the easiest approach is to create a full backup, extract it, and inspect the files to determine where usernames, passwords, databases, and users are stored. Writing an importer based on that usually only takes a few hours, so it is worth doing when you have 100+ accounts to import..
If you end up creating a script, feel free to share it so others can benefit. And if you run into any issues, just post here 👍