Unlike the RSD the average is not stored in the Calibration table so you can't select it as a data field.
It is possible to calculate it in a PDF Report Builder report by way of expressions and fairly sophisticated Python code however it is not at all easy. There are two basic approaches.
#1 Load all the data into a global Python data structure as the rows are displayed in the report and then perform the average calculation once all rows are available. This limits the report format fairly seriously.
#2 Using the SDK directly access the rows of the calibration table, perform the average calculation and store them in a global data structure, and then using an expression relive the average where and when you want in your PDF Report Builder template.
The above is only a framework to a solution not a complete "how to". If you want to pursue this, knowing what is required in terms of skills and time, let me know and I can supply more information.
We are planning to add aggregate functions like average, mean, minimum, maximum, etc. to a future release of PDF Report Builder.