Modifying Promotion recommendations

Hi,

I have to do some modifications in the way Siebel recommends product promotions based on current order line-items. In this way, I already know the workflow related to the process of promotion recommendation: ISS Promotion Recommendation SubProcess. There is a step (Recommendpromotion) in that workflow which invokes the method "RecommendPromotion" from the BS "ISS Promotion Management Service", which, I believe, is responsible to generate the "Recommendation List" based on the current line-items. The problem here is that I cannot edit that method's code, because is from Siebel Vanilla right? Any ideia how I could edit the Product Promotion recommendation?