Martin,
If you have fields in JomSocial for "First" and "Last" name, you can easily import into those fields using the SocialProfiles - JomSocial plugin. If you want the user to be able to see, edit, or enter new profile information, you'll need to switch the Registration Mode setting of JFBConnect to "Normal" instead of "Automatic". That will allow you to show fields during the registration process, either from their JomSocial profile or Joomla itself. There are lots of options to show/hide the fields based on different conditions (if a value is being imported, if one is being auto-generated by JFBConnect, if it's required, etc).
As for the user ID on the social network, that information is available in the Usermap area of JFBConnect.
I hope that helps explain, but if you need anything else, just let us know.
Thanks,
Alex