Hi,
To add an option here it require to edit core files, so changes are lost when updating unless you do class overrides.
The file to edit is "administrator/components/com_hikashop/types/discount.php", in that file you can add the entry as a new 'select.option', then you will have to edit/add javascript to change the displayed fields.
To add fields, the view to edit is "discount / form" in backend template.
Could you give us more details about what you want to add ? Because it will require a lot of other modifications if you want a new discount type...
In your case the best solution will maybe to use a plugin based on custom fields or on a specific action.
Here is documentation about plugins:
hikashop.com/support/support/documentati...r-documentation.html
In all cases, it require good PHP knowledges.