This'll do it:

WITH MEMBER [Accessories]

AS

CASE WHEN [Product].[Product Line].CURRENTMEMBER.MEMBERVALUE = 'Accessory' THEN "Accessory"

ELSE "Non Accessory"

END

SELECT

{[Accessories],[Measures].[Internet Sales Amount]} ON 0,

[Product].[Product Line].[Product Line] ON 1

FROM

[Adventure Works]