We have never tried to install Petals ESB on a AS400 mainframe.
You might have to:
I think it is possible to install a JVM on AS400.
There is no native libraries in Petals to port
Then you need to declare the required packages in a AS400 module something like that.
Then there is the EBCDIC stuffs with documents/parsings/transformations...don't know if there is some work to do to process properly the documents in the Petals JVM.