Filtration by Additional Field

Hi, the additional filed are amazing, useful and i can do a lot of thing.

I have to ask to you if it's possibile to filter the news by a specifical addtional field

Exmpale

<?php

$Category = "15";
$template = "default";
$additionalfield = "value"

?>

How i can do because if i write the name of the additional field nothing happen