$allPreviousVars = ???; // Could be 20+ vars
echo "<a href='".$allPreviousVars."&newVar=2'>Link</a>";

<?php echo http_build_query(array_merge($_GET, array('foo'=>'bar', 'foo2'=>'bar2')); ?>

$query = mySanitizeFunction($_GET);
$url = http_build_query($query) . '&newVar=2';