Additional custom Omega breakpoint CSS files
I found out how to do it, you can do it like this in the template.php file of your theme

// Adds the Delta responsive stylesheets
$theme = alpha_get_theme();
$layouts = $theme-&gt;grids['alpha_default']['layouts'];
$keys = array_keys($layouts);
for ($i = 0; $i &lt; 3; $i++) { 
    drupal_add_css(drupal_get_path('theme', 'MYTHEME') . '/css/CUSTOM-' . $keys[$i] . '.css', array('group' =&gt; CSS_THEME, 'weight' =&gt; (120 - $i), 'media' =&gt; $layouts[$keys[$i]]['media']));
}


This will now look for your own CSS files called 


/css/CUSTOM-narrow.css
/css/CUSTOM-normal.css
/css/CUSTOM-wide.css


And call them in the same way your other breakpoints are called, and if you're looking for the ability to add your own separate stylesheets separate to the normal ones, this'll do it.