Can I add calculated field/column on Excel pivot based on OLAP cube?

Cannot Create Calculated Fields in Excel Pivot Tables If Data Is From OLAP Cube  - November 15, 2010

If you want a calculated field in one of the olap cubes you should use MDX expressions. These are defined on the server and are not too difficult to use. The results of the calculations then form part of the OLAP results and can be included in your pivot tables.

No, you can't. See:
Differences between OLAP and non-OLAP PivotTables in Excel
http://support.microsoft.com/kb/234700
especially on "Calculated Fields and Calculated Items" section
* UPDATE * It can be done with Excel 2010 or later (see "EXCEL HOW-TOs" post)