It is not possible to add Customer Specials to Shared Baskets. When a customer opens the Shared Basket and they chose to purchase items which means the spend requirement is met, then the Customer Specials will automatically appear in the basket and the customer can select them. Customer Specials are not meant to be available on shared baskets as the promotion can change and that would then mean that the Shared Basket would no longer be available if we have moved onto new customer specials since the basket was created.