If you can read that field's value through a Payroll Operation (in a pcr), then you should be able to transfer it's value into an Information WT.
If you can't read that field's value though a Standard Payroll Operation, then I would suggest that you talk to an ABAP resource and ask that a Custom Payroll Operation be created to do that.
If the Payroll Results Table has an "Exemption Amount WT", then it should be easy to show it on teh Renumeration Statement (PaySlip).