You can do it like this: first, just take top N (lets say top 5 members), after that take top 10 members except top five. After that take top 15 members except top 10, and so on...
Here is the formula for the first 5, ordered by [SomeAmount], which is one of the measures from the cube
SELECT
{ [Measures].[SomeAmount] } ON COLUMNS,
EXCEPT(TopCount([Date].members, 5,[Measures].[SomeAmount] ),TopCount([Date].members, 0,[Measures].[SomeAmount] )) ON ROWS
FROM [cube]
and for the next five (first five are skipped):
SELECT
{ [Measures].[SomeAmount] } ON COLUMNS,
EXCEPT(TopCount([Date].members, 10,[Measures].[SomeAmount] ),TopCount([Date].members, 5,[Measures].[SomeAmount] )) ON ROWS
FROM [cube]
And so on...
If you have AdventureWorks cube you can try this query there:
SELECT
{ [Measures].[Internet Order Count] } ON COLUMNS,
EXCEPT(TopCount([Date].[Calendar].members, 10, [Measures].[Internet Order Count]),
TopCount([Date].[Calendar].members, 5,[Measures].[Internet Order Count] )) ON ROWS
FROM [Adventure Works]
And also, for the example from your comment where you have cross join:
SELECT
{Department.members} on COLUMNS,
EXCEPT(TopCount({[Product].[Status].members}*{Date.[Calendar].members}, 10),
TopCount({[Product].[Status].members}*{Date.[Calendar].members}, 5)) on ROWS
FROM [Adventure Works]
You can try this on Adventure Works also.
And also for your example from the comment:
SELECT EXCEPT(TopCount({dim1.members}*{dim2.members},10),
TopCount({dim1.members}*{dim2.members},5)) on ROWS,
{dim3.members} on COLUMNS
FROM [cube]
Hope it helps.