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]