There is community povided solution:
https://assets.zabbix.com/files/zabconf2017/constantin_oshmyan-zabbix_agent_emulator_for_as400.pdf
https://share.zabbix.com/operating-systems/ibm-i-i5-os-os-400-for-ibm-system-i-as-400/zabbix-agent-emulator-for-as-400-platform
But the agent is old version (and not officially supported)