You can't have 2 separate Facebook apps running on your site. It would be really problematic overall.
The whole permissions system of Facebook is really what you're looking for. If a user declines the extra permissions, they're profile won't be filled out with that data. If they accept, it will. Therefore, you don't really need an additional system to do so.. it's already the user's choice.
Beyond that, we don't have and don't plan to support a more granular permission system.
I hope that helps explain,
Alex