How can the Linux Digi net snmp agent snmpd be extended with a custom MIB?

We wish to extend the Linux Digi net snmpd agent with a custom MIB. We would like to extend it using shell scripts via ‘pass’ snmpd.conf option, however I think that the net snmpd agent supplied with Linux Digi does not support ‘pass’?
If it is not possible to extend the agent using shell scrips then can the snmpd agent be extended by compiling a dynamically loadable module in ‘C’ and load via dlmod?
If so what version of net snmp should we use?
The net snmpd agent is Version The latest version of net snmp is 5.7.3, there does not seem to be a version 5.4.2.