I managed to achieve the result by overriding the validate_button script when validating the stock picking: 

def button_validate(self):
        for picking in self:
            if picking.purchase_id:
                picking.purchase_id.state="draft"
                for move in picking.move_ids_without_package:
                    if move.quantity_done > 0:
                    	​po_line = picking.purchase_id.order_line.filtered(lambda l: l.product_id == move.product_id)                                                	​	​	​	​if not po_line:
                       	​	​order_line_id =picking.purchase_id.order_line.create({
	​                	​	​	​'order_id': move.purchase_line_id=order_line_id.id,
	​	​	​	​	​	​	​'product_id': move.product_id.id,                        	​	​	​	​	​		​'name': move.product_id.display_name,                                	​	​	​	     'product_qty_uom': move.quantity_done,                                	​	​	​	​'product_qty':move.quantity_done,                                                           'product_uom': move.product_uom.id,                                                         'date_planned': picking.scheduled_date,})

	​	​	​               picking.purchase_id.state="purchase"        
        
	​	​res=super(StockPickingInherit,self).button_validate() 
		​​return res

before validation I modify the state of the purchase order to draft, I then add purchase order lines with the extra products that are in the stock picking but not in the order and finally I revert the state to purchase. It is important to link the stock picking line with the purchase order line through the stock.move.purchase_line_id field.