appending the values of fields from ztable to vbap

i have 3 fields in my ztable zvxxallocobj. i have to append the values of these 3 fields into Vbap table.using the user exit

USEREXIT_SOURCE_DETERMINATION .

how can i write logic for this inside the user exit.

plz provide me some inputs.

VKORG,VTWEG,SPART,MATNR,KDGRP,KUNNR are the primary keys in my Ztable.

any rought inputs will be enough

Regards.