Coupon for one free product

Hi,

I would like pointers on how to modify nop to support a coupon for one free product.  I'm using nop 1.90

My discount setup is:

Discount Type: Assigned to Category
Discount Requirement: Must be assigned to customer role
Discount Limitation: One time only
Use Percentage: Yes
Percentage: 100
Start Date: 4/1/2011
End Date: 4/1/2012
Requires Code: Yes
Coupon Code: CP100

This setup applies the discount to all products in the category as long as the user is in the defined role "MEMBER".  

What I would like is a feature to limit the coupon be only applied to one item from the category.  So if they add 10 to the card they will only get the item of lowest value free.  

Please let me know if I can supply more information.

Thanks