Is LDAP the right path to go down?
Definitely not! Unless vastly increased complexity, resource usage, and significantly slower performance are your goals (there are situations for which LDAP is an appropriate technology, but this is definitely not one of them). LDAP is merely a user data storage mechanism, like /etc/passwd. It just provides the ability to include arbitrary data and provide network-wide reliable access to that data. This is not a situation where “network-wide reliable access to” user data would be useful…Webmin has access to the local user data already. We’ve just never had folks wanting to grant existing users access to virtual servers, so it’s not in the form.
Or is what I'm looking for even possible?
Probably. I don’t know for sure, though, and I’m not sure if I know exactly what you’re trying to do. There is an “Import” option that allows you to import various types of existing virtual server (like from Apache VirtualHost configuration) and turn them into full-blown Virtualmin accounts. But if you just have a username, that’s something I don’t think we’ve ever been asked.
But, if it is possible, here’s probably how you’d do it:
Forget the “owned by” bit. That’s for creating virtual servers owned by existing virtual server owner accounts (this is the Sub-server account type).
Create a Webmin user for this user, and set it to authenticate to the UNIX password database (this would be PAM or /etc/passwd…but you don’t need to care about that).
Then, in Virtualmin, go to the create virtual server form, and fill in the username in the "Administration username" (and fill out the rest of the form).
See what blows up. Come back and yell at me about it.
In case it isn’t apparent, I don’t know what this will do, exactly…but I would recommend you try it on a test user first (and have backups…you always ought to have backups of everything).
If this doesn’t work, maybe removing the user and recreating it from Virtualmin is the way to go.