The Business Process Flows mainly consists of Stages, Steps and Workflows. When defining a Step you can link it to an existing Field in the entity.
The question I would like to ask is: can you define a Step which is NOT linked to an entity field? I am using an on-premise CRM installation and, unless I am missing an obvious way, the built in editor does does not allow this.
Unfortunately No, there should be a minimum one Data step (Field) atleast in a stage.
It's meant to use for stage-gating using field (data step) for advancing the flow stage. Maybe you can add some non-mandatory field & hide it using javascript.