Well LANDesk only captures Manufacture. Name and Number. I don't think WMI digs that deep to collect the serial number, so I think you are stuck trying to find a utility from the vendor that will expose the S/N from a command line. I think this may be a hard ER to request since there may not be a standard across all scanners to obtain S/N
What you can do is get WMITools from Microsoft and then browse and see if WMI returns the serial number. If it does then you can use a script or some method to retrieve the serial number and put it into landesk.