How to insert a row in Tablix dynamically based on runtime values?

Hi All:

I'm new to ReportViewer and Tablix and need some help. It is a Visual Studio 2010 ASP.Net web report project. I have a tablix/ReportVeiwer with , say, two columns--High School Name, Total Students; Group by: City of a given state. And I got it working with sub total for each city.

Now the question: if I want to add a new sub total row ONLY for two cities X and Y, how can I do that?

Thanks a lot in advance.