Hy, it is not possible with just some default registry key as i know, but first of all you can use
the plugin winuser to idenitfy all users
https://plugins.ocsinventory-ng.org/
I haven't used this plugin, but it looks like the plugin can't identify the admin users. so can can create a script to do this
in a batch and a english windows version you can use something like this
net localgroup administrators
to write it to a share u can code something like this
net localgroup administrators >>\\Server1\thisisasharename$\%computername%_filename.txt
you can write the output to a network share whre only "computer objects" have write rights.
Hoppefully the plugin will be extended to list admins and for example deactivated accounts which you can retrieve with this command
wmic useraccount where "disabled=1 AND LocalAccount=1" get name
and activated accounts
wmic useraccount where "disabled=0 AND LocalAccount=1" get name
Kind regards