Spanning a Tablix column

Question

I am using Microsoft Reporting Services (rdlc, that comes with Visual Studio 2010)

Suppose I want to create a report that has a Group Header, detail rows, and a group footer. (I'm using Tablix, but if there is a different way, I'm open to it).

Now, there are several columns, but I want for the group header to span the columns. Is that possible?

Another question: Suppose I want to conditionally span columns in my detail row, is that possible? (and how?)