There are two ways I can think of to accomplish this.
1) You could edit jboss-portal.sar/conf/hibernate/user/setup.txt. This file defines the initial set of users. With a decent editor with macro capabilities you could very easily expand this file to create any number of initial users. The only caveat is that this would work only on a clean portal (one that have never been run before).
2) You could user JMeter or Grinder to create the users. Use the proxy capability of either tool to record a session where you create one user, then edit the script to generalize the user id. And the run the script.
I have done both #1 and #2. #1 I used to create a handful of initial users. #2 I use to create a large quantity of users, usually naming them user-N, where N is 1 through 9999. Makes it easy to write performance tests to randomly select a user.