40 Hour work week instead of Annual Hours

ADP client that is moving to GHR for the HR portion.  They are remaining on ADP Payroll.  They use a "standard hours" field in ADP for the number of hours worked.  It can be anywhere from 1-40.  They do not use annual hours to calculate pay rate.  Has anyone ever created a field or used IPA to do a calculation to derive hours/week.  I suggested a user field that they populate and pass over to ADP, but they don't like the fact that the FTE and Annual Hours are then incorrect.  Any suggestions?