You can do this if you have the netcat utility on your system. It basically involves converting your desired list of users/passwords into a set of Citadel server commands, and then batch uploading them into the system. Create a file named create_users.txt that looks like this:
USER your admin name PASS your password CREU user name 1|password1 CREU user name 2|password2 CREU user name 3|password3 . . . CREU user name 99|password99 QUIT
Of course, "your admin name" is the login for a user who has Aide level access and can create users. Now put these server commands into the file mentioned above -- and netcat it into the server like this:
(cat create_users.txt; sleep 10) | nc your.citadel.host.org 504
You'll see a bunch of messages coming back at you that say:
"User 'foo' created and password set."
That should be all it takes! And you don't have to stop there -- if you want to take the time to learn more about Citadel protocol commands, you can basically batch-input anything you like. Webcit operates on top of citserver via that protocol.