There isn't a way to add a custom field to the Project Overview section, however, you can change the order of the Project Details area so that the custom form appears first followed by the Overview. You can do this by clicking on the pencil icon (edit) on the right side and the drag / drop the custom form above the Overview.
Another way around this would be to create a project report that lists out the stakeholders for each project and share that with the team for easy reference. You can add that report to a dashboard, and add that dashboard as a custom section on the project as well, as the results will dynamically filter based on the object in which you are viewing.