Adding your own MIB objects (so called ManagedObjects) to your SNMP agent with SNMP4J-AgentX works in the same way as for SNMP4J-Agent. The only exception from this are AgentX shared tables, but these are advanced concepts you generally do not need when you start using AgentX/SNMP.
Thus, I recommend reading the SNMP4J-Agent-Instrumentation-Guide.pdf to learn how MIB objects are registered and instrumentated based on your requirements.