You could possibly utilize the Except function [EXCEPT( )]. You'd probably want to create a named set for the top 10 initially, then use that as the excluded set of items (using the except function) to create a calc member. So in rough pseudo code....
With
Set setTop10 AS
Member allElse AS SUM(EXCEPT( )
SELECT
{setTop10, allElse} on 1,
{[Measures].[Qty_sold]} on 0
FROM
[some_cube]
Steve.