I don't think you could do this in the standard sellerdeck setup. AFAIK the sellerdeck discounts functions are best at doing financial promotions such as discounts, which can be 100% on a product, but I don't think it ever got around to adding products to the cart automatically.
You could do this in perl by adding the free product to the cart cookie on setup (or it might even be possible in javascript if it's a fixed product).
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