Add support for the AVERAGEIF function:  https://support.office.com/en-us/article/AVERAGEIF-function-faec8e2e-0dec-4308-af69-f5576d8ac642

Returns the average (arithmetic mean) of all the cells in a range that meet a given criteria:

AVERAGEIF(range, criteria, [average_range])


Workaround: This could be implemented as a custom function: http://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/features/formulas/custom-functions.html