I'm not sure if you're asking, but I think it would just be something like this (existing code starts around line 34, I added the part in red):
Code:
// #############################################################################
// slurp all enabled feeds from the database
$feeds_result = $vbulletin->db->query_read("
SELECT rssfeed.*, rssfeed.options AS rssoptions, user.*, forum.forumid
FROM " . TABLE_PREFIX . "rssfeed AS rssfeed
INNER JOIN " . TABLE_PREFIX . "user AS user ON (user.userid = rssfeed.userid)
INNER JOIN " . TABLE_PREFIX . "forum AS forum ON (forum.forumid = rssfeed.forumid)
WHERE rssfeed.options & " . $vbulletin->bf_misc_feedoptions['enabled'] . "
AND rssfeed.rssfeedid IN (1, 2, 3)
");
where of course you'd change the 1, 2, 3 for each file. You'd also want to disable the original rssposter.php task.
I don't have time to set things up to test it, but if you want to try it, I think it should work.