How support for @media inside a media => "..." should work.

foobar.css

.a { color: red; }

@media (max-width: 600px) {
	.a { color: blue; }
}

.b { color: green; }

Output.css

@media screen {
	.a { color: red; }
}
@media screen and (max-width: 600px) {
	.a { color: blue; }
}
@media screen {
	.b { color: green; }
}

resources.php

<?php
array( 'foobar.css' => array( 'media' => 'screen' ) );