Add in back-end new custom attribute for affiliates

Hi there,
In my site is not permitted registration of members, only admin can add affiliates. I wish I could add custom attributes in the section “Manage affiliates”. Currently I can only add: “Paypal email, Refferal Amount, Amount Refferal method, Refferal Rate, Coupons,” I need to add new fields, which should not have any function, only need to be written.

Thanks