Bonus Buy creation for Site Group

Hi Friends,

We have an new requirements for creating bonus buy's.

Currently We have three key combinations in our process:

1. Sales org./sales chan/site/bonus buy/article/sales unit.
2. Sales org./sales chan/price list/bonus buy/article/sales unit.
3. Sales org./distr. channel/bonus buy/article/sales unit.

Now our requirement is that we need to create bonus buy for site groups (one Bonus buy for Big Stores and other one for Small Stores). Can you please guide me how to create bonus buy for site groups.

Thanks in Advance

Shankar.P.P.