I need to add some extra fields (start date and end date) in Banner
add/edit page (admin->cms->banner).
If possible I want to create a new extension for adding my fields.
You could look at using an observer to add some custom fields to the banner if the module allows you to. In the banner module you should see
But i doubt the module will have this.
Or something similar to this.
Alternatively there is an open source module that has event for allowing you to add custom fields like the ones you want. https://github.com/DoghouseMedia/Doghouse_Carousel