Hi Karanika,
It is not possible to define the exchange rate diff.based on WBS in OBYC for KDM. However GR/IR clearing account also not define based on WBS, but it is based on valuation class maintain in the material master.
Moreover, you can try to define a substitution for this requirement. You have to define a substitution in GGB1 as mentioned below and later you have to assign to your co.code in OBBH.
Prerequisite : BUKRS = XXXX and HKONT - GL account no which is defined in KDM and PROJK = XXXX
Substitution : HKONT = GL account.
Like that you have to create various steps for each WBS element or group of WBS elements for different exchange rates. First do testing in Dev.
Regards,
Mukthar