I would use a combination of eCart, DataAssist and security assist for this.
in your users table you will need a User Status column. set the default value for tyhis column to 0
use eCart to add the ability for the user to purchase the registration.
when the transaction is complete, use data assist to update the User Status column to 1.
in the Security assist Authenticate user behavior on the login page, it will be configured to compare the username and password columns on the entered values.
Add another comparison to only allow login if the userstatus column is 1.
