UNION a final ROW in MDX

Question

Hi, I'm quite new to MDX and im having some trouble getting the following t-sql query to MDX.

select distinct
  System
from Systen
  where System <> 'MIS'
UNION
  SELECT  'ALL'

So far i got something like this. But i have no idea how to add that final row 'ALL'.

SELECT 
  {} ON COLUMNS,
  {[Concesionario].[Sistema].[Sistema].ALLMEMBERS} ON ROWS
FROM 
(
  SELECT 
    -
      {
        [Concesionario].[Sistema].&[]
       ,[Concesionario].[Sistema].&[MIS]
      } ON COLUMNS
  FROM [DW]
);

Any idea how to do this?

Thanks