I apologize but i am not sure i fully understand the question.  If you want to add custom fields to a customer record, you would turn on the custom fields in the “My Field Labels” area of company preferences (requires the pro version or multi-store).

But since you also mention discounts, […]

Resolution for 'add fields issue: 'How do I add/edit custom fields? When I select a customer, apply a discount, how do I add fields for where to assign the discount to?' available: Yes.

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