I don’t believe we have a command line tool to do this, but you can probably use HTTP Posts to do the same commands that the web reports does. You can look at the HTML source on the web reports user management pages to see what values they are posting when you add/change/delete users. Alternately, you can try to adjust the users table in the besreporting database directly.
The password is hashed with a specific method that I will try to look up… In any event, you can always make a script for the HTML POST calls that I mentioned if you want to do this.