[eluser]rogierb[/eluser]
It can probable be done using your apache config and .htaccess.
The second domain should probable have its own rootfolder where a index.php and .htaccess resides.
The .htaccess then should route to index.php/username.
You basically insert the 'username' part manually as the first segment.
Another way is to use the same root and adapt the index.php:
<code>$_SERVER['REQUEST_URI'] = '/username'.$_SERVER['REQUEST_URI'];</code>
but this will become unmanageable if you have a lot of domainnames/