(i.e add &userid=X for all posts by userid X or &username=Y for all posts by user Y).
File Edits:
In showthread.php find:
if ($threadedmode == 0)
{
Add after:
// EDIT ########################################################################
// get posts by a specific user only
if (!empty($_REQUEST['userid'])) {
$usersql = 'AND post.userid = '.intval($_REQUEST['userid']);
$uidwhere = true;
} elseif (!empty($_REQUEST['username'])) {
$fetchuserid = $DB_site->query_first("SELECT userid FROM ".TABLE_PREFIX."user WHERE username = '".addslashes($_REQUEST['username'])."'");
$uidwhere = false;