[jdh]

The only way to do this, unfortunately, would be to create a custom Applescript that runs on a scheduled basis (e.g. through launchd) to poll the mailbox looking for flagged messages to add to OmniFocus. As somebody else has already pointed out, Mail rules only trigger on new mail as it's coming in, not on anything that's already in the Mail app (even marking messages as unread doesn't re-trigger the Mail rules).

I used to have a kludgy script that did this with an "Action" folder in an older version of Mail... Similar workflow in that I would move messages to the "Action" folder and then the script which ran every five minutes would pick up the message and add it to my OF inbox. I'm using Gmail rather than Mail.app these days and the script is probably buried in my backups somewhere now, but it wasn't a huge modification from the various Mail-to-OmniFocus scripts that can be found around the forums -- basically it just needs to be modified to search Mail instead of being trigger from a Mail rule.