I've got some experience of using JPA with the AS/400, which I hope will be of some help.
The choice between SEQUENCE and IDENTIFIER comes down to whether you can modify the existing table or not.
If you are in a position where you can alter the table, then you can add an identifier column to it, like so:
then, in your java entity have the following:
If, on the other hand, you're not in a position to alter the table, you can create an SQL sequence to apply an identifier for you.
On the AS/400 you'll need to enter in:
And then in your java entity have the following:
You'll probably want a separate sequence for each table.
Hope this helps.