SSAS / MDX Query - Add an extra dimension when using Order
I'm not quite sure what you're asking, but I think you want to do the following (correct me if I'm wrong):


You want to see a breakdown of sales by month, one row for each month, with a total for all months at the bottom.
You want to see the items across the columns, ordered by the item with the largest sales volume to the smallest. 


select non empty
       order ([Item].[Item].Children
             ,([Measures].[Total Line Value]
              ,[Date].[Month].[All])
             ,BDESC)
    on Columns
       non empty
      ,[Date].[Month].Children
    on Rows
  from [Sales Analysis]
 where [Measures].[Total Line Value]


The columns are each individual item, sorted by the total Line Value over all months, i.e. from all-time best seller to all-time lowest seller, with items having no sales at all excluded.

The rows are the months for all time where data exists.  You can use a set definition to specify any date range you want.  Note that this assumes the existence of a dimension called [Date].

This query will show the items in order of total sales across the columns and the sales for each month by item down the rows.  I'm not 100% sure if this is what you want; feel free to clairfy your question if I'm not on the right track.