It depends. What OS do you have running on the AS/400? OS/400? Or have you actually installed something like AIX on it?
If you've got OS/400, you can enable SNMP ( check http://www.redbooks.ibm.com/abstracts/sg244504.html ). Once you've done that, you can load it into Hyperic as a Network device, or write your own custom SNMP plugins to get specific OIDs that you'd like to monitor in Hyperic. You can also ( just found this out myself ) send traps from the AS/400 to Hyperic.
If you've installed AIX on it, have you tried installing an agent on it? Not sure if it would work as I've never dealt with the AS/400's, just pSeries and some of the newer xSeries.