As the dialog says, it uses the local/domain user accounts, not a text file like htaccess. Which I believe is typical for Windows.
As this is a security issue, the actual login/password's are most likely stored somewhere safe, which probably is a file (certainly not the registry), but in an encrypted form, not as ascii.
I don't know of any utility to dynamically import a login/password text file into a domain, so you probably have to either program some sort of interface yourself, or administer it with the normal GUI tools. Or install Apache as a parallel webserver.