you will need to hand code the logic to accomplish this.
create a new blank page called something like redir.php and set the login redirect to go to the redir.php page:
"successRedirect" => "redir.php",
On the login page, you can double click the authenticate user serer behavior, go to the third page of the authenticate user behavior and set the user name column to be stored in a session variable, the new session variable will use the same name as the column, so if the column is "username" the session variable will be too.
then on the redir.php page, you can hand code a header redirect to use the username session:
<?php
if(isset($_SESSION['username']) && $_SESSION['username'] != "") {
header("Location: ".$_SESSION['username']."php");
}
?>