you could use php to do the trick
example:.
<?php
echo "<marquee>" . include("YourExternalFile.html") . "</marquee>";
?>
or you could use mysql and periodical update the field in the table
<?php
$con = mysql_connect("MysqlHost","Username","Password");
if (!$con)
{
die('Could not connect: ' . mysql_error());
}
mysql_select_db("my_db", $con);
$result = mysql_query("SELECT * FROM news");
while($row = mysql_fetch_array($result))
{
echo "<marquee>" . $row['marqueetext'] . "</marquee>";
echo "<br />";
}
mysql_close($con);
?>