User Registration Add-On is only triggered by a form submission. So you can't retroactively apply the User Registration Add-On to existing entries. It only happens when a feed is created for a form and then that form is submitted from that point going forward. You can't apply it to existing entries.
You would have to find a plugin for importing users to your WordPress site and then you could export your existing entry data to CSV and then import them using another plugin.
Posted 8 years ago on Monday June 20, 2011 | Permalink