Da gibt's das SQL-Register

CURRENT USER
bzw.
CURRENT_USER
oder einfach
USER

Also

update wrktest1/plz_001 set (F2, F3, F6, plzben) =
(select F2, F3, F6, USER from
wrktest/plz_998 where plz_001.F1 = plz_998.F1)
where plz_001.F1 in(select plz_998.F1 from wrktest/plz_998)
and F2 = 'BERLIN'