DBA Setup to Track Preferred Rate

Has anyone had experience setting up a negative deduction for a preferred medical rate?  We would like for the employee to choose the medical/dental plan in a self service type enrollment (via a third party), where the preferred rate is not evident.  Based on completing required documentation, the Benefits staff will update a Category Code.  We would like for this to prompt an additional DBA (negative amount) to reduce the total amount of the deduction, which can not be chosen or manipulated by the employee.  Additionally, our medical plans are not a flat rate, but rather based on a salary