Adding custom field to a payment method plugin

Hi;

I want to add a custom field named "MoneyOrderDiscount" to CheckMoneyOrder payment method's Configure view. But I couldn't decide how to get this value inside application.

There is a method inside IPaymentMethod "GetAdditionalHandlingFee". I think to add a method just like it named "GetMoneyOrderDiscount". But I should add this to any other payment plugin then.

Do anyone has an idea about how i can handle this issue?