A post about AS400 from 2009 on these forums.
FileMaker can perform ODBC imports from other software providing you have the correct driver from the software manufacturer of the source of the data (AS400). You can setup your ODBC Data source in the FileMaker file if you wanted to script an ODBC Import.
That said, you cannot relate that ODBC Data source to your FileMaker tables unless it is one of the supported External SQL sources. The ESS feature allows you to create relationships and pull live data from a number of SQL sources, and you can find more information on that here: link
In short, you cannot relate an AS400 data source to a FileMaker table. For more information about ODBC you can visit: link