Using solely workflows, what you are asking is for is not possible.
You could use rules based pre-event triggers to only allow Headings to go to Draft or Finished, while allowing a Functional Requirement to tranistion only through Draft to Review, Finished, Revision, or Deferred. Please see the section titled "Creating Workflow and Document Event Triggers" in the Server Administration Guide appropriate to your release of Integrity.
Kind Regards, Kael Lizak
Senior Technical Support Engineer PTC Integrity Lifecycle Manager