Sorry for the non-response previously! I actually did do some research on this, but then never got back to you as I didn't get all the info I needed.
For your question, right now, now, we don't have a way built-in to JFBConnect (or JLinked) to sign up for a newsletter during registration. Before, I was looking for a Joomla User plugin that may extend the user registration form to add the signup you were looking for. We execute the Joomla User plugins during registration, so any that work with built-in Joomla registration *should* work with JFBConnect/JLinked as well. Unfortunately, we haven't tested any of the extensions in the JED for doing this.
Oddly, on a side note, probably the best MailChimp integration extension, JoomlaMailer, isn't listed on the JED. Not sure if I just missed it, or it's not listed for some reason. It is free, and (I thought) GPL, so not sure why it's not there. Either way, a link to it is:
joomlamailer.com/
They have a registration plugin that adds the signup to some forms, but I don't think it uses the normal way to extend Joomla forms, so don't think it will work immediately with our extensions. That was what I wanted to test earlier, but somehow that task got lost.
Hope that helps explain a bit. We are planning to overhaul our add-on system for profiles so that it's much easier to add fields and do other things during the registration process. That revamp should be coming next year and, hopefully, allow us to add stuff like this easier (or 3rd parties to add it easier).
Thanks, and sorry again for the delay,
Alex