I think that the tsung framework can actually add the users for you. But I have not done that.
I’‘m actually using the Openfire JDBCUserProvider and JDBCAuthProvider, so I was able to add thousands of fake users that way. It might be something to look into, as it’'s fairly easy to work with and just requires you to write some TSQL.
One note: for tsung, the users need to be named in a uniform fashion, ie user1, user2, etc. And their passwords also need to be uniform, ie password1, password2…
Tsung basically loops over an integer range and appends the integer to a static string for username/password, which you set in the tsung.xml.