Hi Srinath,
For default value, use TMG event - 05.
A work around is to write a report and run it as periodic batch job
loop the table and
loop ztable into lwa_ztable
lwa_ztable-field4 = sy-datum.
lwa_ztable-field5 = sy-uname.
MODIFY ztable from lwa_ztable.
ENDLOOP.
Thanks,
Anil