My colleague tried to add workflow to WFFM forms recently, and discovered that you cannot. We filed this with Sitecore support and we've been told it has been forwarded to the product team.
Unfortunately, this means that advanced users that edit a form are directly editing the approved version of a form, and there is no way to create a 'draft' version.
The support portal seems to be down right now, so I can't extract the ticket number, but I'll update this once I can.
Update: This issue is logged from us as ticket #405553. It's currently 'closed' due to inactivity, but if you reference it Sitecore should be able to track down the current status with the product team.