Can a flexfield be added to the Award Funding train stop?

Looking at the possibility of adding a flexfield to this popup window where funding is added or edited in the Funding train stop for Awards. I located the Grants Funding Source Descriptive Flexfield, but I'm not finding where that exists in the UI to expose it. Does anyone know if it is possible to add a flexfield on this particular page? We'd want to add the example in the red box that we mocked up.

Thanks!
Jon