Samba (SMB) is an add-on connectivity tool, and there aren’t any defined standard SNMP MIBs for tracking its utilization which rules out making a custom probe.
Monitoring the process is usually done on the SMB/CIFS server itself. There might be a Nagios NRPE plugin available to do it at https://exchange.nagios.org/directory/Plugins/System-Metrics/File-System/SMB
The user would need to put the NRPE script on the server to be monitored, then use the Servers > Proprietary > Nagios NRPE probe to run the script on the remote server.