Hi and Welcome to the Board,
I haven't worked with connections to Quickbooks, but you should be able to create a calculated field.
It's not clear what you mean by "the middle of my Pivot Report"....Calculated fields need to be placed in the Data (or Values) area the Pivot Report, but there is no restriction on the position you place the Calculated Field within that area.
There are some scenarios in which Calculated Fields cannot be used, such as when also using Grouping.
Have you used Calculated Fields before with PivotTables that reference simple workbook ranges?
If so, what happens when you try to create a Calculated Field using the same process?
If you need some more help, please identify what version of Excel you are using.