Adding a new Workflow Type requires the addition of a number of new supporting artifacts. It also requires changes to existing artifacts, namely the form and the table that is associated with the record for which the workflow type is being created. There are properties that must be changed on the form to enable it for Workflow and you cannot make these changes via an Extension. Instead, you must customize via Overlayering.
The Application is going to be sealed at some point which will prohibit customization via Overlayering. This request is to allow Form Property changes as extensions.