Yeah, never thought of that. While there's not a configuration option for that right now, here's how you could do it yourself if so inclined.
In the following file: /plugins/authentication/jfbconnectauth.php, update line 68 to whatever name you'd want to use:
$response->username = "fb_".$fbUserId;
In your case, changing it to the following should do what you're looking for.
$response->username = $fbUser."_".$fbUserId;
PLEASE, make a backup of this file after you change it. Installing a new version of JFBConnect will overwrite the file and undo your change (shouldn't affect any users already created), so with each installation you'd need to re-do this change. Sorry for that inconvenience. We'll try to add a configuration option in an upcoming release.
Also, please test and report any issues you find!
On a side note, I'm glad to see you're using the pseudo-user feature. We're always curious to see who is, as it seems like most people aren't.
pseudo-Joomla username
