The Python function _check_sum is called to check that you have distributed all costs over all products:

https://github.com/odoo/odoo/blob/10.0/addons/stock_landed_costs/models/stock_landed_cost.py#L137

Create your own module that replaces this function with one that doesn't check the distribution:

class LandedCost(models.Model):

     _inherit = 'stock.landed.cost'

    def _check_sum(self):

         """ Check if each cost line its valuation lines sum to the correct amount

         and if the overall total amount is correct also """

         prec_digits = self.env['decimal.precision'].precision_get('Account')

         for landed_cost in self:

             total_amount = sum(landed_cost.valuation_adjustment_lines.mapped('additional_landed_cost'))

             if not tools.float_compare(total_amount, landed_cost.amount_total, precision_digits=prec_digits) == 0:

                 return False

            # val_to_cost_lines = defaultdict(lambda: 0.0)

            # for val_line in landed_cost.valuation_adjustment_lines:

                # val_to_cost_lines[val_line.cost_line_id] += val_line.additional_landed_cost

            # if any(tools.float_compare(cost_line.price_unit, val_amount, precision_digits=prec_digits) != 0

                    # for cost_line, val_amount in val_to_cost_lines.iteritems()):

                # return False

return True