In the Dynamics CRM 365, we've customized the Business Process Flow specific to the needs:
As shown in the screenshot, the Source stage is nesting 2 fields:
Subject
Description
We wanted to customize these.
From where we can update these Stage specific details?
Stages cannot contain Subgrids, you can put fields (making them mandatory or not according to your needs).
Business Process Flows being Processes, they can be edited either from SETTINGS -> PROCESSES or through solutions (which have a 'Processes' element).
Finally, as always when it comes to customizing, docs are on MSDN