At the moment, Deployment Manager supports only 4 environments in a CICD lifecycle.
We have plans to support more stages in one of our upcoming releases.
In the meanwhile, why don't you consider tweaking your workflow to have 4 stages itself. Our research suggested that 4 stages are generally enough to have your app change go through enough testing cycles.
Please let us know what your specific use case is for 6 environments.
One other suggestion is to have multiple pipelines, for example you can have a Dev pipeline that stops in QA and a separate release pipeline or as many additional pipelines as needed (see the image below). As noted however, a future version of Deployment Manager will support more than 4 stages per pipeline.