If you want the upsell to be added to the original purchase before it is completed that would be something you would need to build into your website before buyers actually get sent off to PayPal to finish their purchase
For example, your first "buy" button leads to your upsell page where the buyer can then choose between a buy button that represents your product plus the upsell, or use a buy button that is just the product itself. From there they continue the purchasing process as normal.
