There is no pre-registration in dap.
So the customer comes in, clicks on the paypal button, completes payment, gets added to dap automatically.
Now if the customer to update any custom fields you have in the dap profile, then just redirect these users upon payment to their profile page so they can update the custom fields and other profile fields before they proceed.
If you want the users to be redirected to a profile page upon payment, then you can add a redirect tag to dap generated button code:
<input type="hidden" name="redirect" value="http://yoursite.com/myaccount" />
Pls see this on how to add custom fields and other fields to dap 'FREE' signup form :