First step, you never want to modify Kentico's existing items. Make sure to CLONE the web part first (Files and object).
Next, on the cloned object in the Web Parts module, go to the "Fields" tab and add your field.
Then, modify the cloned ASCX webpart file to do the logic you need to.
https://docs.kentico.com/display/K9/Customizing+web+parts
https://docs.kentico.com/display/K9/Developing+web+parts