when using the extend feature of net-snmp the tables afterwards is indexed by the name of the command. So you can ask i.e. for:

nsExtendResult."mycommand"

which returns the return value of the command you configured in the extend section of the agent configuration.

with snmpget you use it like:

snmpget <host> nsExtendResult.\"mycommand\"

Please note the esacping of the " in the command.

When I configure an item in zabbix with a OID .nsExtendResult.\"mycommand\" there is an error in the server log and the item is switched to "not supported".