So first things first: to create custom fields for an OOTB Portal Widget, you would need to do a few things:
1. Find the widget you want to create custom fields for on the sp_widget table.
2. Click the "Clone Widget" UI Action on the Widget record, or inside the widget editor (I believe it is a menu selection in the editor).
3. Find how and where the fields are being generated in the widget and how the info is being passed client to server or how it is working. Add your field.
Generally, you are not able to create a custom field on a ServiceNow-created widget without cloning. Some may allow it via instance options try ctrl+click on the widget.
Which widget are you trying to customize? Do you have any screenshots or other info?
