reportingservices-2005

Can I add a row to a matrix based on specific rows in the matrix rather than all?

Hi, I have a matrix in a report that returns five rows. effectively this r1 | s11 | s12 | s13 | s14 .. r5 | s51 | s52 | s53 | s54 I need to add a sixth row that computes a value based on the values in row 1 and row 5. r6 | s11+s51 | s12 + s52 | etc.... Is this possible and if so, how do I do it? ...