There really isn't an easy method to do exactly what you're trying to do. Our profile import stuff is a lot more rigid than we want it to be, and at the risk of sounding like a broken record, it is something we're planning to improve over the next few releases for doing some of the non-login importing I've been mentioning.
For now, if you know what fields your importing from the user, you can call:
$fields = array(0 => 'first_name', 1 => 'last_name', 2 => 'status', 3 => 'email', 4 => 'name');
$profile = $this->getUserProfile($fbUserId, $fields);
$value = $profile['last_name'];
You could import any field, but the list we support is in /administrator/components/com_jfbconnect/models/config.php, starting at line 143. There are some fields which need some extra logic to pull out the real data, and they have a strange format in that file (like hometown_location.city). In that case, you'd do:
$value = $profile['hometown_location']['city'];
It can get confusing fast, but this may hopefully help you.
Hope that explains a little. Should you need more info, if you wanna go down this route, let us know, and we'll help how we can.
Alex