HI Lobna,
First of all I am in total agreement with Rémi Corriveau, that you should endeavor to see if there is a SAP standard solution available for your requirement it this is a legal requirement, if not and if this is a custom requirement raised by your client. then what I have proposed should work.
CRT (cumulative results table) carries the WT from RT and cumulate it for various purpose esp YTD values carried by various WTs based on their characteristics.Check processing class 20 for your WT.
you can call CRT value of a particular WT in a PCR using AMT=CYXXXX or subtract value of CRT of a WT by using SUBWTCXXXX or add value to CRT by ADDWTC.
CRT values are restored to zero at the end of the tax year so you will then have to transfer your CRT values into first pay period by writing a PCR to call the Q value of a WT in OCRT table after function SETCU within sub schema of XLR0.
I did mention you have to be little hands on if you wish to achieve the solution using PCRs. should you wish this solution can also be achieved by an ABAPer by a custom function.
KR
Sid