create variable using the settings>variable overview option for each of the expressions.. like Var1 will have the expression of Billed, Var2 will have the expression of Backlog... and similarly the others.. and.. create a new expression and add all the four variables.. $(Var1) + $(Var2) + ....
hope this helps..
If you just want to add values from all 5 columns and then create a 6th column,
say,consider the second row in the pivot table,
70,000+30,000+ 0 + 27,000+0
then just write this in the expression for 6th column as
Column(1)+Column(2)+Column(3)+Column(4)+Column(5), you will get the result as $127,000.
But if you are looking for something else, I assume you will need to explain you problem further.