Add an initial bonus to an affiliate

No, that's not possible. Actually, the money earned by the affiliate is calculated based on all the orders/leads/clicks attributed to that partner which are not yet paid. You could do it manually by creating an order in the database in the table hikashop_order for that partner and setting the amount earned in the order. But to do that automatically with an option will require to handle the credits of the user and recalculate it when a new order is made. Then, it would be possible to set a default value for that amount.