Would someone please help us on how to set up extra 10% commission for our affiliates in April only?

We are running promotion in April to give our affiliates extra 10% commission. Our current commission rate is up to 15% (default 10% plus up to 15% by sales volumn). By adding extra 10%, it will give our affiliates up to 25% commission. This promotion is for April sales only, and after April it should be back to the original setup. How can we setup the rate structure in ShareASale admin control?

Thanks in advance. Any help is appreciated.