Add the selection of Suppliers in the Promotion selection so if we have a promotion based on a particular supplier we can use the promotion feature to set it up.
have a look here, they are building me a custom promotion in part based on this
Addons :: Tools :: Promotions - discount on products from a supplier(s) (cs-cart.pl)
Please pay attention to our solution
https://www.ecom-labs.com/ready-to-use-add-ons/cs-cart-promotions-for-suppliers-add-on.html