We discovered this capability when trying to publish a WFM service in an environment where no WFM admin client was readily available (Firewall restrictions, etc).
Using the arcgis server admin API, we selected an existing WFM service to edit. (Home/services/myservice/edit) Copied and modified the JSON to suit the new environment.
Then went to the create service page in the new environment (Home/services/createService) and entered the modified JSON, selected JSON as the format.
Selected Create, and the service was created/published just as expected.
While we didn't find documentation to support this explicitly, I presume that this is supported, as long as you copy a similar service, and modify the JSON correctly for the new environment.