DB Connect (https://splunkbase.splunk.com/app/2686/) is supposed to be able to connect to DB2 database. I haven't tried this particular database myself but it's the general solution for RDBMS connectivity. You might have to fiddle a bit with particular JDBC configuration to make it work.
Thank you for the response!
Yes, but I could see that DB Connect support DB2 only when it is hosted a Linux box but not AS400
From Splunk base https://splunkbase.splunk.com/app/2686/:
Splunk DB Connect supports DB2/Linux, Informix, MemSQL, MySQL, AWS Aurora, Microsoft SQL Server, Oracle, PostgreSQL, AWS RedShift, SAP SQL Anywhere, Sybase ASE, Sybase IQ, and Teradata.
Disclaimer - I never used DB2 so I have no idea if there are or aren't any huge differences between differently hosted databases. But I'd test it anyway. DB Connect after all is just a client app for the database. If you can connect with the database client to a DB2 on Linux, why shouldn't you be able to connect to AS400 one?
Doesn't cost you more than a bit of your time to test it 😉
If I recall right, there could be something special with DB/2 on AS400/iSeries/OS400? There seems to be some instructions how to use/connect with jdbc on net. You should just use google to find those.