[Neo]

Find

PHP Code:

    if ($thread['postuserid']) {
      $thread['postedby'] = "<a href=\"member.php?action=getinfo&userid=$thread[postuserid]\">$thread[postusername]</a>";
    } else {
      $thread['postedby'] = $thread[postusername];
    } 


and replace with

PHP Code:

$secondname_db=$DB_site->query("SELECT fieldx FROM userfield WHERE userid='$thread[postuserid]'");
    $secondname=$secondname_db[fieldx];
    if ($thread['postuserid']) {
      $thread['postedby'] = "<a href=\"member.php?action=getinfo&userid=$thread[postuserid]\">$secondname</a>";
    } else {
      $thread['postedby'] = $secondname;
    } 


edit the fieldX to whatever the field number is.

Have fun.