How to include "Additional Compensation" in custom report?

I can include "fixed salary (100%) in custom report, but the salary is often more complicated than that, so we define "Additional Compensation" (payroll custom fields) for various employees. I didn't find a way to include those attributes in custom report, which means there is no way for me to export them (with time axis). Am I doing something wrong, or is it a bug?

I really need to be able to export the employee compensation for our employees using the API.