The server-side OfficeWriter renderers for Reporting Services use ExcelTemplate and WordTemplate to populate the report and stream it back to the user. However, since Reporting Services does not provide runtime control, it is not possible to post-process the output of SSRS with ExcelApplication inside of SSRS.
However, In version 9.1.0 new functionality was added to open ExcelApplication and call Workbook.CalculateFormulas() on all templates after running them through ExcelTemplate by adding keys to the Reporting Services web.config. See below for more details.
Adding the Key to Web.Config
To set the key on the Server go to web.config located in \Reporting Services\ReportServer. Add the following code to post-process all of your output with Workbook.CalculateFormulas()
Put the code before the closing of the configuration tag, as in the image below. Make sure not to place it inside any other tag. If web.config already contains an appSettings section, add the keys to the existing section. Do not create another appSettings section. You can also have keys that set ExcelTemplate and WordTemplate properties in this appSettings section.
Note: The keys are case sensitive so make sure True is capitalized. The default value of these properties is False.