Idea to add custom fields to All Items view. It was requested with https://support.freshrelease.com/support/discussions/topics/50000000510 and it would be address in the near future.
By being able to add a custom field to the view it helps scenarios when you are working with other parties who maintain their own board/backlog and you want to reference their unique identifier to the Freshrelease item.
We have enabled Custom String fields in the List View. We will support other data types in the future