How to add additional information to the closed opportunity form

Hello community, 

I want to be able to collect additional information for when an opportunity is closed; however, the close opportunity form is locked and uneditable.  I'm wondering what the best way to do this would be and how.  My thought is:

1)  Trigger a dialog to appear when the closed opportunity form is loaded (or saved?)

2) The dialog would ask/allow for the user to select one or more of a multiple list of reasons for the opportunity closed.

3) The information would somehow be attached to the closed opportunity record.  Would this require creating a custom entity that has opportunity as a parent and forcing the dialog record to that entity?

As always, your help is appreciated.