You can easily make a Php script to parse your old htaccess, I am using this one for PRestashop rules :
$content = $_POST['content'];
$lines = explode(PHP_EOL, $content);
$results = '';
foreach($lines as $line)
{
$items = explode(' ', $line);
$q = str_replace("^", "^/", $items[1]);
if (substr($q, strlen($q) - 1) !== '$') $q .= '$';
$buffer = 'rewrite "'.$q.'" "'.$items[2].'" last;';
$results .= $buffer.PHP_EOL;
}
die($results);