What I'm actually trying to do is to set up an "Administrators" group that simply has access to everything that all Virtualmin's users have and nothing more. I suspect it won't be possible to do this precisely, but the closer I can get the better.
What I'm struggling with right now is some bulk updates. For example, to give the Administrators group access to all MySQL databases that Virtualmin's users have access to, I can't use the All Databases option because that gives access to databases not managed by Virtualmin. The Selected option is the desired one, but here I need to specify each Virtualmin user's database manually, and keep the list updated as Virtualmin accounts are added, removed, or modified. The same issue comes up with Apache log files, and PHP .ini files for example - they should be specified manually and kept up to date.
So what is the best way to automate this? I took a look at the /etc/webmin directory's *.acl files and believe I can write a script to do it through there, but I don't find all the Virtualmin users there (actually find some that don't exist, but I think it has something to do with an account that was renamed), and haven't found a spec for how to safely navigate these files, so not sure if this is the best approach...