You need to look at Trxn PC00_M99_CIPE (or something like *.CIPE) with pgm RPCIPE00 that is used for posting pay results to FI/CO (not The transaction PCP0 linked to program SAPMPCP0 is to edit the posting runs).
The posting pgm has Exit 001 goes to function module EXIT_RPCIPE00_001, to get modified KOMOK(Symbolic Account) and MOMAG (EE Grouping for Account Determination).
Exit 002 EXIT_RPCIPE00_002 in the posting pgm is to obtain modified ZUONR (Assignment number).
It also has an exit 003 to import other pay results.
I believe the exit 002 may be able to do what you need. However you should note that the C0 or C1 assignment number with the details (like CC, fund) should have been created during the payroll through the payroll schema/rules based on wage type and any other specific needs. You may also need to add an entry to C1 table through a custom operation. The person’s RT entry for the wage type is to be altered with c1znr equal to this assignment number. You can look at the C1 table for an emp in the RT, you can see the different fields for each assignment number and the RT will have the C1ZNR for the related wage type.
The normal costing would come through the person’s cost CC in InfoType 27 or in the person’s position’s cost distribution details. The person’s IT 27 would over-ride the position’s details.
Hope this helps. Good luck!