How to load data to new keyfigure

Dear All,

I am given a requirement , Please Help

i am asked to include a new KF (Number KF) in the cube

the cube is getting loaded by data source 0CO_OM_OPA_6(in between DSO)

in the data datasource there is field BELNR which loads to 0CO_DOC_NO in cube

where the logic of loading to KF  should be as follows

check the  field value BELNR in table COBK( Ex : 223931661)

check in the same table COBK   for field value REFBN(Ex :905283981)

THEN

go to table BKPF check for the field value BELNR

IF

COBK(REFBN)  Value = BKPF(BELNR) Value ( Here Ex :905283981)

THEN

GET Value of KURS2 from Table BKPF in to key figure in Cube

A small demo

TABLE                                                        TABLE

COBK                                                          BKPF

Fields                                                           Fields

BELNR  = 223931661                                BELNR  = 905283981

REFBN = 905283981                                  KURS2 = 0.50672( in to cube)

Please Advise

Thanks in Advance

Harshitha