I think that I tried something with the rest API. It was a script that did this and only a user that is already an admin could do it (again all this I did them with a PHP script that used the rest API).
Sorry Benouare that I did not see your message earlier!
The main web interface refers to “Group Admins” - but I think that terms is the same thing as “subadmin” which is often used.
I have not been able to find anything in the occ interface which is related to Group Admins. Possible there is something available on an option I have not tried yet. I get impression that not every possible feature is documented. OCC Commands in Admin Manual
However there does seem to be something available, tho not used it myself,
Link below documents the functionality of a completely different interface to “occ”.
Rather it seems to be part of the “User Provisioning API” which is based on URL queries which return XML data.