[Kirk Y]

Custom fields aren't queried for in the threadbit template, so you'll need to use a plugin to hook them in. Give the following a try...

Add a new plugin using hook location "forumdisplay_query" with the following contents:

$hook_query_fields .= ' , userfield.* ';
$hook_query_joins = "LEFT JOIN " . TABLE_PREFIX . "userfield AS userfield ON(userfield.userid = post.userid)";

Add a new plugin using hook location "threadbit_display" with the following contents:

if ($thread['fieldX'])
{
$thread['postusername'] = $thread['fieldX'];
}

Just change the X above to the Profile Field's ID and the User's actual username will be switched out with the nickname, if they've specified one.