I'm not sure if version 6 has the capabilities to do this. You can have a separate person be the approver, but not based on a flag. It may be possible to override the event hook to (based on a flag) move it to a specific 'locked' step, but that may not be around till later versions.
May want to seriously consider upgrading to the higher versions or rebuilding the site, the newer versions have much more control (With the EMS license) to do advanced workflow steps.