This is only possible to do using specific OIDs that contain this information and a custom OID sensor if this information is available for the device through SNMP. With many network devices, they will have an OID that contains the serial number but this OID can vary from device to device. You would have to look at the MIB provided by the manufacturer to find this info.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.