Addiding existing data

I am trying out this plugin as it seems to offer the best solution to my requirement. I need to allow a registered user to add a post with a number of custom fields, maintain their post and pay a charge for this.

I also need to allow other users to register, but only to see the site, and the posts other people have added. Finally, all non registered users can see the rest of the site.

I am creating 2 forms, one for users to add their post, and the other for users who just want to register to see the posts.

However, I have many hundred of existing users. Is there a way off adding these?

Is it possible to achieve the above?

Regards
Pete

http://wordpress.org/plugins/wp-user-frontend/