till you get an answer from Foto50, here is some:
read this :
wiki.jopensim.com/index.php/plg_jopensimregister
once you have activated this plugin, you need to decide what new users have to do , mandatory opensim registration or not. community builder, as well as this plugin come on the top of the registration system.
After regarding your existing users, depending on the number, you will have figure out a work around, example you may allow them to create an opensim user by adding this facility to a private menu or something. Well if yoou have like 1000 or more users, i think you have no choice.
Lets see what Foto50 would say