Helder,
JFBConnect creates completely normal Joomla users, so if you already have a subscription management tool in place, JFBConnect shouldn't change the process much. Once they log in, they could still purchase their upgrades and log in with that Facebook/Joomla account.
You'd have to rely on the tools in your subscription component to manually change a user's level, but it should show all Facebook user's in the same way it shows the Joomla users.
Hope that helps, but if you have more information or more specific information on the configuration and extensions you're using, that would help.
Thanks,
Alex