Since you have AS400 systems, I'm assuming you're entitled for ibm support, so you may wanna go for ClientAccess and have it installed on your standard ed sql. If you have 64 bit system, there is a patch you need to apply before you install clientaccess.

Whether you wanna upgrade to sql enterprise and use db2oledb, or use clientaccess is up to you.

There are other software out there for sure that serves the same function so you may wanna search if you haven't done so.