We have a user authentication system for phpNuke; however, we do not have "blocks" (not usre what they are" and Classifieds is not a "module", although we provide mechanisms to try and use it as such. We don't have a very active phpNuke group of users, so other than making sure our system lets users be authenticated, we havent done much beyond that. phpNuke users have posted in the forums about what they've done to integrate, but, again, its not a very active group.
I would say it depends on just how integrated you want it and weather you are willing to put in a little effort.
Please do not PM me for support or sales questions. Thank you for your understanding.