In Promotions > Discount Rules you can create a rule that automatically (or with a coupon) adds a product to the customer's shopping cart.
On the Coupon Information page you can specify if this rule will apply to specific customer groups, automatically or with a coupon, between certain dates, or how frequently it can be used.
On the Conditions page you can build the logic that governs when the free product is added to the cart. For instance, if the user has a specific product in their cart (see below example)
On the Actions screen you can configure the below settings:
Set this drop-down to Automatically Add Product To Cart
Enter the SKU that you want automatically added to the customer's shopping cart.
NOTE:Only products of type Simple, Virtual, or Downloadable are allowed, and the product cannot have customizations.
Enter the quantity of the product that you want automatically added to the cart
When set to Yes, customers can edit the quantity of the added product in the cart.
When set to Yes, customers can remove the added product from the cart.
Enter the discount percentage you would like applied only to the product automatically added to the cart.
0= no discount on the automatically added product, the user will have to pay full price
100= 100% discount, the automatically added product is free