I guess I need a bit more information about how it should work. Has the newsletter signup ever been part of your normal registration flow, or is this something you're trying to do? As for posting the data, is there simply a URL you have to post to.. like:
If so, what happens after the data is posted there: is the user redirected somewhere, or is this something can be done silently in the background?
There's a few options I can see to do this. The simplest, though more manual, is to simply add that field to Community Builder. Then, we can help you display that field during registration (Create a select box with "Yes/No" for signing up, and mark it required.. then, in the Profiles area of JFBConnect, set it to show non-mapped required fields. Once submitted by the user, periodically, you can dump out the users that have that set to 'yes' and import to your mail software.
Depending on your answers to the top, we should be able to help you automate it as well (you'd create the field the same, and we can help you create a user plugin to auto-submit the newsletter subscription).
Either way, we'll help you get setup however we can.
Finally, definitely hunt me down at Joomla Day Austin! Great to hear others we work with will be there. I'm talking for about 30 minutes or so, but will be there the whole day. Can't wait!
Alex