Hi Kian. If I understand correctly, you want to add custom fields using contact extender.
The Contact Extender plugin adds fields such as First Name, Last Name, Title, Department, Office Phone, Mobile Phone, Contact Email, and Contact Address.
Although results labels are customizable, I don’t see a way to customize the field labels for contact extender.
So I think the answer is, no, contact extender cannot be used as a custom fields add-on.