Gracias a los dos por contestar, lo que he echo es lo que me proponia TMeister.
Agregando la función a /themeurl/functions.php y modificandola ahí.
Código PHP:
<?php
/**
* Overrides the Wordpress sanitize_user filter to allow special characters
*/
function sanitize_user_modificada($username, $raw_username, $strict)
{
//Strip HTML Tags
$username = wp_strip_all_tags ($raw_username);
//Remove Accents
//$username = remove_accents ($username);
//Kill octets
//$username = preg_replace ('|%([a-fA-F0-9][a-fA-F0-9])|', '', $username);
//Kill entities
$username = preg_replace ('/&.+?;/', '', $username);
//Remove Whitespaces
$username = trim ($username);
// Consolidate contiguous Whitespaces
$username = preg_replace ('|\s+|', ' ', $username);
//Done
return $username;
}
add_filter ('sanitize_user', 'sanitize_user_modificada', 10, 3);