Cryil,
What you're looking for is generally done automatically by a user plugin for the extension that you want. The user plugin detects a new registration and adds whatever data is necessary to their own tables.
You can try to re-order the Seblod user plugin (assuming there is one) to after the JFBConnect - User plugin to see if that helps. Beyond that, I don't have a good answer, but I'd recommend checking with Seblod on how they handle registrations from 3rd party extensions. JFBConnect uses the standard Joomla registration process and calls the same triggers for creating a new user that Joomla does, which is what all extensions are supposed to use.
I hope that helps,
Alex