If all you need to do is add that extra parameter, just concatenate it on to the end of $args
. (As a side note: I recommend using an array for args instead of the query string method as I find it much more readable.)
<?php
....
$args="&posts_per_page=".$numreviews.'_reviews&order=DESC&orderby='.$feedsort.$metakey.'&author=123';
....