add column MDX

I'm new to MDX, trying to learn, does not seems easy as SQL.

I extract data from a cube within excel :

I would like to add a column that would be :  COL2 - COL3

I know I need to go in OLAP Tool, create a MDX Calculated Member or Calculated Measures.... but I don't know how to formulate my MDX Query. Everywhere I find some sample of MDX Query they are all part of a complete select statement which does not help to understand.

I just want a calculated column :  Col2 - Col3  .  How do I do that ?

here is the underlying MDX query :

SELECT NON EMPTY Hierarchize({DrilldownLevel({[Dim Sélection Période].[Par Sélection Période].[All]},,,INCLUDE_CALC_MEMBERS)}) ON COLUMNS,
      NON EMPTY Hierarchize({DrilldownLevel({[DimTypeBudget].[Budget des ventes].[All]},,,INCLUDE_CALC_MEMBERS)}) ON ROWS  
   FROM (
          SELECT ({[Dim Sélection Période].[Par Sélection Période].[Périodique].&[60],[Dim Sélection Période].[Par Sélection Période].[Périodique].&[50],[Dim Sélection Période].[Par Sélection Période].[Périodique].&[30]}) ON COLUMNS ,({[DimTypeBudget].[Budget des ventes].&[6],[DimTypeBudget].[Budget des ventes].&[5],[DimTypeBudget].[Budget des ventes].&[4],[DimTypeBudget].[Budget des ventes].&[3],[DimTypeBudget].[Budget des ventes].&[2],[DimTypeBudget].[Budget des ventes].&[1],[DimTypeBudget].[Budget des ventes].&[0]}) ON ROWS  
          FROM [Canac DM VentesClientMag])
          WHERE ([Measures].[MontantVente])