I do not think it is possible right now.
Perhaps you can go to support.dotnetnuke.com and enter it as a feature request.
I would like this too.

What you are trying to do is pretty easy once you set up the security roles for a subscriber ( if you dont know how to do that, go to "Security Roles"  in the Admin and select "Create an new Security Role" )

Pretty straight forward you would think but it does not add the option to join that role on the registration page, something I really think DNN should include in a future release. Right now this is what must be done, once a user is registered they must log in, they will be a registered user, they must then click on their user name (usually in the top right corner area), this will take them to their user profile, on that screen they will see a link called "manage services" they click this and they will then see the subscription role you set up, they click the role on are wisked away to paypal to pay and then brought back to your site when its done. The nice part is that Paypal does update your user accounts upon a successful payment recieved. Thats it in a nutshell. This is why in seaching for a more USER FRIENDLY way. Until that is found, I have altered some the Access Denied.RESX file to tell the user where to go to access the subscription and have set the user login to the subscription role page to force the Access Denied page to come up this is the best I have come up with so far. Good luck, its not easy, especially as a newbie. I have only been at this for about 6 months myself.