I'm not sure if the EXCEPT operator is available in DB2 V7 on z/OS, but if it is, you can try the following query --
select * from (select * from emp fetch first 5 rows only) as temp1
select * from (select * from emp fetch first 2 rows only) as temp2
This query would give rows 3 thru 5. An assumption made is that the table has a primary key or the column-list given by SELECT * forms a unique key across table. This would ensure desired results given the functioning of EXCEPT.
I have tested this successfuly on DB2 V8.1 on Unix.