Hey Joshua,
There are several mechanisms for bulk user creation, including command line tools found in the /usr/libexec/webmin/virtual-server directory.
Here's the docs:
http://www.virtualmin.com/support/documentation/virtualmin-commands/Specifically, the create-user.pl command is documented here:
http://www.virtualmin.com/support/documentation/virtualmin-commands/ch04.html#id2538022You could, pretty easily, wrap this command up in a bash for loop to create users from usernames and passwords found in a file. If you're unsure how to do that, give me an example of what your source of usernames looks like and I'll see what I can come up with.
If you run into any problems, feel free to query us here or in the customer issue tracker. Or, if you think you find a bug, in the bug tracker.