I would approach this by using the SQL in your report (preferably the stored procedure that the report calls).

WITH myTop5 as (

select top 5 customer

from myTable

group by customer

order by sum(salesAmouns) DESC

)

SELECT t1.customer, sum(t1.salesAmount) as totalInSales

from myTable as t1 inner join myTop5 as t2 on t1.customer = t2.customer

group by t1.customer

union all

SELECT 'All the others', sum(salesAmount)

from myTable t1

where NOT EXISTS (

select t2.customer

from myTop5 as t2

where t1.customer = t2.customer

)