I don't have much insight into exactly why you need the utf-8 conversion task, except to say that SQL server - I believe - uses UCS-2 as its native storage format, and this is similiar to UTF-16 which is what your task converts the file to. I'm surprised SSIS can't work with a UTF-8 input source though.
My main point is to answer the "How could I replace this script task with a more elegant solution?":
I have had a lot of success using HiT OLEDB/400 Server. It allows you to set up your AS/400 / iSeries / System i / whatever IBM are calling it this week as a linked server in SQL server, and you can then access the 400's data directly from the server its linked to using the standard 4 part SQL syntax, e.g. SELECT * FROM my400.my400.myLib.myFile. Or even better, it's much more efficient as a passthrough query using EXEC...AT.
Using this you would not need SSIS at all, you'd just need a simple stored proc with that does an insert into your destination table direct from the 400 data.