Coloque * em subdomÃnios no cPanel para configurar o curinga
Coloque isso em .htaccess
Options +FollowSymLinks
RewriteEngine On
RewriteRule ^([aA-zZ])$ subslug.php?username=$1
RewriteCond %{HTTP_HOST} ^(^.*).yourdomain.com
RewriteRule (.*) subslug.php?username=%1