Your question envolves a lot of technical things. I think you should be more specific and segment your initial question in many others, by example:

It is the same database structure?
Are you using a marketing joomla plugin and do you want your data matches joomla’s database structure?

What is the As400 server rol in this case? it is only database server?

I have a lot of questions, but maybe you prefer to rewrite your question, so we can help you better.

The as400 is database only server.
Would use marketing joomla plugin if it was recommended … yes data to match structure, I assume.. Sorry don’t know much more technical on this .. kind of need a kick start in the right direction

Ok, first you need to know if the database structure is the same in both servers, if so then you can do a data dump or export to a text file using a program like DBVisualizer or similar. The resulting file has commands for database recreation and the insert commands to write your data too, in this way you only need to run the commands in the other database to recreate your database.

If your database structure is not the same then you need to write a program to do the migration of data doing the right thing to save the data in the correct way. This task is for a programmer.

If you database structure is not the same, you need to know the database structure and system functionality to work in the website design like the last one task.

Other way is if your database structure is the same then you need to change only database connection parameters to your joomla installation and then you will have your new website up and running.

Hope these few lines helps you a little more.

yes they do clear things up a bit, thanks.